The car brake bible - how brakes work, including disc brakes, drum brake, calipers, hoses, brake fluids and general brake maintenance, current and future brake technologies, DIY car maintenance and much more.

The Brake Bible

Power Brakes and master cylinders.

Power brakes (also known as power assisted brakes) are designed to use the power of the engine and/or battery to enhance your braking power. Whilst you can generate a fair amount of force using your foot, using systems from elsewhere in the car to help you apply even more force means that you get more powerful brakes as a result.
The four most common types of power brakes are: vacuum suspended; air suspended; hydraulic booster, and electrohydraulic booster. Most cars use vacuum suspended units (vacuum boosters). In this type of system, when you press the brake pedal, the push rod to the master cylinder opens a vacuum control valve. This allows vacuum pressure (normally from the intake manifold) to "suck" on a diaphragm inside the vacuum assist unit. This extra vacuum suction helps you to produce more force at the pedal end of the brake system.

Hydraulic booster systems usually utilise pressure from the power steering system to augment pressure on the master brake cylinder.

Electrohydraulic booster systems use an electric motor to pressurize the hydraulic system downwind of the brake pedal which has the effect of amplifying the internal pressure in the whole system.The advantage to this system is that as long as you have battery power, you have power brakes even if the engine fails. With vacuum-assist brakes, no engine means no assistance.

If you're curious about how power brakes work, go out to your car and with the engine off, step on the brakes. They'll have a slightly solid, almost wooden feel to them. Turn the engine on and do it again and you'll notice a lot less back-pressure on the pedal. This is the power assist which is making it easier for you to depress the pedal.

The components of a master cylinder.

The main components of a typical master brake cylinder

Brake master cylinders are complicated affairs involving finely manufactured parts, minute tolerances, springs, o-rings and rubber seals. The diagram here is a simplified representation of a dual-circuit master brake cylinder. When you step on the brake, its connected to the main plunger (on the right side of this image). As this is pushed into the master cylinder it acts on the components inside. The rear plunger (in blue) is the first one to start moving. As it moves forward, brake fluid from the reservoir is sucked in through the fluid intake and return port. At the same time, fluid is sucked in through the equalisation port. As the second circuit rear seal passes the intake and return port (about 1.5mm after the plunger starts moving), it creates a fixed volume of fluid between the rear and front plungers. The more you step on the brake pedal, the more this fluid is now forced out into the second brake circuit to apply those brakes. At the same time, the pressure building up in this area overcomes the strength of the first circuit return spring and the front plunger (red) begins to move too. As with the rear plunger, it too sucks fluid from the reservoir until the first circuit rear seal passes the fluid intake and return port (again about 1.5mm), trapping fluid between it and the front of the master cylinder. This fluid is then forced out into the first brake circuit, applying those brakes.
When you take your foot off the brakes, the return springs push the plungers back into their neutral position. Fluid returns to the brake fluid reservoir and the system goes back to an unpressurised state.

One last thing about brake master cylinders: they used to cost an absolute bomb to replace. Historically if your 20 year old beater developed a leak, it was probably cheaper to buy another used car than to replace the master cylinder. Nowadays not so much. The internet changed all that in the mid to late 90's with online parts stores, which drove prices right down. Now you can pick up new master cylinders for $200 or so. That's a price break which is cheap enough that it's silly to get your leaking one remanufactured when you can just grab a new one.

Cross-linked brakes - why there are two brake circuits.

In the rendering of the master brake cylinder above, you'll see there are two plungers and two brake circuits. This is the most common design for cars today. It's a form of redundancy in the brake system. The idea is that only two brakes, one front and one rear, are on either of the brake circuits. For four brakes, you therefore need two circuits. But why? Well imagine one of your brake lines springs a leak - for the sake of argument, the front-left brake. If all four brakes were on a single circuit, when the master cylinder began to pressurise the brake system, fluid would spurt out of the broken line and pressure would never build up. In turn, that means none of the brakes would ever come on and you'll sail merrily into the back of the vehicle in front of you.

An example layout of cross-linked brakes

Imagine the same scenario with two circuits. As the first circuit pressurises the front-left and rear-right brakes, fluid spurts out of the broken line and those brakes are never applied. However because the master cylinder is also pressurising a separate second circuit connected to the front-right and rear-left wheels, those brakes do apply and you've still got braking force. Sure, it's reduced, but it's a hell of a lot better than no brakes at all. Because of the front-left to rear-right and front-right to rear-left linking of the brake circuits, this type of system is known as cross-linked brakes. The rendering here shows an example arrangement of cross-linked brakes.


A word about handbrakes.

It's worth spending a moment here to talk about handbrakes. Or parking brakes, e-brakes or emergency brakes depending on where you come from. Whilst they're good for doing handbrake turns, they're not especially effective at actually slowing you down. They will - don't get me wrong - but you won't be seeing any stellar performance out of them so the term 'emergency brake' is a bit of a misnomer. So why is this? Well, handbrakes are cable-actuated for a start so the amount of power they have is wholly dependent on the amount of tug you have in your arm. There's no hydraulic system to help you out. Apart from that, they only work on the rear wheels, so you're not getting four-wheel braking. On drum-brakes, the handbrake is connected to a small lever that pivots against the end of one of the brake actuating pistons. When you pull the handbrake, the lever gets pulled and the brake shoes are pressed out against the inside of the drum.
On disc brakes, the handbrake normally works a second set of brake pads in the rear caliper. They're little spots, about the size of a grown man's thumbprint and they're clamped mechanically against the brake rotor. These pads never need changing because they're normally only used at standstill so generally don't wear much. Their small size is the other reason you shouldn't expect stellar stopping performance if you yank on the handbrake. That being said, there are derivatives of disc-based handbrakes that use a mechanical arm to press the main brake pads against the rotor although these are less common as far as I know.

When to use handbrakes

Typically you ought to use your handbrake whenever you're stopped somewhere, be it parked, on a hill or waiting at traffic lights. The reason is simple : if you're parked or stopped, you generally don't want the car to run off without you. At traffic lights, it's an accident minimisation function as much as anything. If you're sitting there with your foot on the brake and someone drives into the back of you, the impact will cause you to take your foot off the brake and you'll go sailing into the car in front, causing more accidents. If you have the handbrake on in the same scenario, your car will largely stay put (apart from the initial shove across the ground as the energy from the impact is dissapated through your tyres). Of course there are personal habits and mechanical complications to contend with here. For example in a car with an automatic gearbox, it's force of habit to just use the footbrake. Even so, you should still use the handbrake when you're parked, especially on an incline. The 'park' setting on automatic gearboxes isn't sufficient to hold a car on a hill, and apart from that, it puts incredible strain on the transmission and clutch system if you let the whole weight of the car transfer into the transmission to try to keep it from moving.
In some American cars, the handbrake isn't a handbrake at all, it's a second footbrake on the far left side of the footwell, which is basically totally useless because it's a pain to put on and even more of a pain to get off because it's a one-way ratchet system (you have to force the pedal all the way down to get it to release). Then there's the ignorance factor. When I went to my new owners orientation evening after buying a Subaru in America, one lady asked what the parking brake was for. (Apparently the name wasn't obvious enough). The dealer representative told her it was a relic of days gone by, not to be used, and he didn't understand why manufacturers even put them in cars any more!

When not to use handbrakes

The first and most obvious answer to this is : when you're going at any speed. If you yank on the handbrake at any speed much over 30km/h, the back end of your car will start to slide. Great for stunts and tricks, not so great if you're trying to stop in 5 lanes of crowded motorway traffic.
The other time you should not use your handbrake is in post-snow, freezing conditions. With the salt and grit that gets put down on the roads, you'll be driving through a salty, snowy slush and it will be spraying all over the underside of your car. If you park and put the handbrake on, you risk it binding on by freezing. Why? Well handbrake cables are almost always exposed to the elements at some point under your car. If you put the handbrake on and the cable is covered in slush, as it freezes again it will lock the handbrake on. There's no solution to this other than waiting for the weather to warm up. Well, not unless you fancy a crack at the Darwin Awards, because some people have tried using blowtorches to thaw the ice, not understanding that they were working right underneath the petrol tank. So here's a tip : don't.
If you need to park in those types of conditions, try to find level ground and leave your automatic gearbox in "p" or your manual gearbox either in first or reverse gears.

Regional variations

One last thing to know about handbrakes : for some reason, from-the-factory settings on handbrakes vary largely with region. In Europe for example, the handbrake is easily capable of exerting enough friction to prevent the engine from being able to move the car from standstill. In America, it's not uncommon to see handbrakes adjusted to lightly that even when fully on, you can just drive off. The only way you'll notice is the handbrake light on the dash, the lack of performance, or the smell of burning as your rear brakes burn off.

Anti lock Braking Systems - ABS

Stop without skidding, and maintain control of the vehicle. That's the premise of ABS. It was first introduced in the 1980's and has been undergoing constant refinement ever since. The system is typically comprised of 4 ABS rings, 4 sensors, an ABS computer and a number of pressure-management circuits in the brake lines. The ABS rings are attached either to the wheels, or more often, to the brake discs. They look like a notched ring - see the image to the right.

A close-up of a brake disc with an ABS sensor ring

The sensors are magnetic field sensors which are held very close to the ABS rings and can detect the slight change in magnetic field as the teeth on the ring pass them. The pulsing field tells the ABS computer that the wheels are spinning, and how fast they're spinning.
When you brake, the wheel rotation starts to slow down. The ABS computer "listens" to the input from the sensors and can detect if one wheel is slowing down much quicker than the others - the precursor to the wheel locking up. (This all happens in milliseconds, by the way). When the computer detects this condition, a pressure regulator in the brake circuit interrupts the pressure in the brake lines by momentarily reducing it so that the brakes release just enough to give the wheels a chance to keep spinning rather than locking up. The computer then instructs the regulator to re-apply full pressure and again measures the wheel rotation. This on/off/measure cycle happens around 15 to 30 times a second. If the ABS kicks in, you'll feel it through the brake pedal as a vibration because the pulsing in the brake circuit affects all the components.

Newer generation ABS systems

As technology marches on, so does the control / feedback system used in ABS. It used to be the case that any single wheel approaching lockup would cause the ABS system to pulse the brake pressure for all the wheels. With the latest vehicles, the ABS computer is connected to 4 pressure regulators instead of just the one. This means it can selectively apply pulsed braking only to the wheel(s) that need it. So if three of the tyres are gripping well, but the front-left is beginning to skid, the ABS can unlock the front-left brake and pulse it to try to regain grip. It's called three- or four-circuit ABS and it's all very James Bond. When hooked up to the traction control system, this type of multi-circuit ABS can also be used to influence the overall traction of a car in extreme maneuvers, such as helping to prevent rollover and inside-wheel-lifting.

ABS and skid control

So how to talk about the biggest misconception about ABS - that it will make you come to a stop more quickly? This is a prickly subject to talk about. In one camp you have drivers like me who just can't stomach the idea of a computer breaking the physical connection between my right foot and the brake system. Whilst in the other camp you have people who believe that ABS is the best thing since sliced bread. It's these people in the second camp who have the all-out belief that ABS will help you stop faster, and in certain conditions, this is true. On a wet or greasy road surface where the traction is severely reduced, an ABS system can pulse the brakes and prevent lockup much better than a human can. But why? The whole point of brakes is to slow you down. To do that they rely on friction in two places - between the brake pads and the rotors, and between the tyres and road surface. If one of those factors is taken out of the equation, the brakes become useless. The most typical situation is that a driver will panic-react to something and step on the brakes with as much power as they can muster. The brake system amplifies this power, grabs hold of the brake rotors and the wheels stop turning almost instantly. This causes the tyres to now skid across the road surface, and as they do so, they become subject to dynamic attrition. In other words, if a tyre is rotating and gripping the road, the "stick" factor is much higher than if the wheel is locked and skating across the same surface. So that's what ABS does - in an emergency, it ensures that the wheels don't lock up but instead keep spinning so that the tyres maintain grip with the road. (That's where ABS gets its name - Anti-Lock Brakes.) This is where the real benefit of ABS comes into play. If you're going to attempt to avoid an accident, the best thing to do is to try to steer around it. If your tyres are skidding on the road surface, you can point your wheels pretty much wherever you want because the actual direction you end up going will have nothing to do with the wheels and everything to do with the direction you were travelling, combined with the camber of the road. Once the tyres lose grip, all bets are off. With ABS, if those wheels keep turning and the tyres keep gripping, then when you ham-fistedly grab the steering and yank it to one side, the car will still turn and you might be able to avoid the accident. So that's the true essence of ABS - to maintain control over the direction of the car.

So why the negativity, Chris?

My bone of contention with ABS is not so much to do with the technology as the placebo effect is has on drivers. ABS is widely misunderstood and if you ask most drivers, they'll tell you that ABS helps them to stop more quickly, and as I illustrated above, in certain conditions this is true. But even the most well-trained driver is going to be subject to panic in an emergency, and more often than not, will lock their arms on the steering wheel bracing for the coming impact. Once you do this, you're no longer steering so the ABS is trying to give you control over your car but you're not taking advantage of it. Given that this is the most natural human instinct, people accept this as "the way of crashes" but somehow believe that if they have ABS, they'll be able to stop before they get to the point of impact, and that's simply not true. I believe too many people think ABS gives them a license to drive faster, because they mistakenly believe that it will get them out of any situation. It's yet another technical placebo that has been put into vehicles which is making the standard of driving worse. The more gadgets and "driver aids" that get put into a car, the worse the drivers become because they live in a rose-spectacled world where they believe that it's the car's responsibility to get them out of any sticky situation that might arise. It bothers me so much I have a "rant" page dedicated to it here : Nanny Cars.

Political correctness and the push for ABS in every vehicle

It's a widely perpetrated myth that speeding is the cause of most accidents, so it follows that if you can develop a method of helping drivers to bring their vehicles to a stop in a more controlled fashion, you'll help to reduce the number of accidents. Good idea, but it doesn't have a lot of substance to it. If you check my page with studies on the facts vs. the fiction of speeding, you'll see that only 4% of all accidents are caused by loss of control of the vehicle with excessive speed as the primary contributing factor. So ABS wasn't really designed for that - it's difficult to reduce the incidence of the already lowest cause of motoring-related accidents. In truth, distracted drivers (like I mentioned above, driving in their cosetted mobile living rooms), their actual ability to drive properly (training and advanced driver courses) and their ability to have some form of spatial awareness are much bigger factors than speed itself and none of those can be overcome by clever braking systems. Shouldn't we be pushing for more driver training programs to attempt to treat the real cause of the accidents rather than simply putting a bandage on the result?
So what about the emotive issue of pedestrian accidents? What if you, the driver, could stop quicker? It's a staggering fact that 84% of vehicle-pedestrian accidents are actually the pedestrian's fault and in most of those cases, even if you could have stopped on a dime, the accident would not have been prevented. Seriously. Read the the facts vs. the fiction of speeding page - you'll be astonished. I'm not condoning running over pedestrians - that would be stupid. I know first-hand what it's like - I had one of those 84% jog out in front of me using his cellphone when I was riding my motorcycle some years ago. I hit him square in the back despite being hard on the brakes, and threw him a good 10 metres down the road. He survived with some scrapes and bruises but I still think about it to this day. I can't begin to imagine what it would have been like if the stupid bugger had actually died.

ABS in snow and ice, and on gravel

abs education alliance

Ah yes. The subject of a good 75% of the emails I get about ABS. The two camps for this argument are split almost exactly 50/50. In one camp, those like me who from experience would rather have their tyres lock up in deep snow to give me at least a fleeting chance of having them dig through the snow to find some road. Those who have anecdotal evidence that ABS is total crap in snow and ice. Whilst in the other camp, those who again believe ABS will somehow magically stop them from crashing in the same conditions. Those who have similar anecdotal evidence disproving all those in the first camp.
ABS by its very nature is designed to stop the wheels from skidding by allowing them to keep turning. On deep packed snow and ice, that's exactly what they're going to do - skid, so ABS effectively removes a considerable amount of your braking in an emergency in these conditions. It's why some cars have ABS disable systems for snow and ice, and it's why ice racers yank the fuse to the ABS system before they even get in a car to race.
The ABS Education Alliance, a group aiming to help educate drivers on how ABS will best benefit them, has this to say on the subject:
Even in fresh snow conditions, you gain the advantages of better steerability and stability with four-wheel ABS than with a conventional system that could result in locked wheels. In exchange for an increased stopping distance, the vehicle will remain stable and maintain full steering since the wheels won't be locked. The gain in stability makes the increase in stopping distances an acceptable compromise for most drivers.
So the short answer to this debate is that ABS is worse in snow and ice for overall stopping distance, but better for controlability.

The hidden gremlin of ABS - what they don't advertise.

If you look at the statistics for crashes, a large percentage of them are "fender benders" - low-speed impacts that only do a little damage and so slow that the vehicle occupants are in no danger; normally about 10mph. I'll give you one guess what the typical "minimum activation speed" is for ABS. That's right. On a lot of vehicles, the ABS is useless much below about 10mph. Seriously. Try it yourself. Find an empty road on a slight downhill grade - even better if its on a dewy morning. Run your ABS-equipped car up to about 10mph and jam on the brakes as hard as you can. The car will skid to a stop and the ABS system will remain totally silent.

Aftermarket ABS systems

To the best of my knowledge, there's no such thing. A few years back a couple of companies tried to market what they called ABS systems that could be retrofitted to any vehicle. The product was a cylinder with a pressure-relief valve in it. The idea was that you inserted this system into the brake circuit somewhere. When you stomped on the brakes - symptomatic of locking up the wheels - the pressure relief valve opened and bled off some brake fluid into the cylinder, thus lowering the braking pressure being sent to the wheels. The idea was to take the "spike" off the initial push of the brake pedal so it wasn't ABS at all. The whole idea of putting something like this into a brake circuit makes me shudder - I wouldn't want to be the person trying to get their insurance and medical claims through after an accident when the investigators found one of these contraptions in their brake line!

A final thought on ABS

Consider this: if you're in an accident and your ABS works perfectly, you'll leave no skidmarks on the road surface. An inspection of the car will show the brakes and ABS system are working perfectly but the absence of skidmarks could lead the police accident scene investigator to believe you didn't brake at all. That in turn could lead to you being the "at fault" driver with all the consequences that involves. Think about it. This exact scenario happens many times every day. Amongst all those ABS-related emails I get, at least one a week is telling me about someone who's had this problem.....

Remember : ABS attempts to ensure that your car stops in the shortest distance possible for most road surfaces. It is not a substitute for you, the driver, paying attention to the road and your driving.

Brake-assist and collision warning systems

Picture credit: Volvo

Volvo Collision Warning System

By 2006, brake-assist and accident warning systems were starting to find their way into consumer cars. I for one just don't like the idea. The manufacturers are reinforcing the misconception that the driver is no longer responsible for their actions. Volvo's collision warning system (CWS), for example, constantly monitors your speed and uses a radar with a 15° forward field of view to determine the distance to any object in front of you. If the distance begins to shrink but you don't slow down, the system sounds a buzzer and flashes a bright red light in a heads-up display to alert you. The brake pads are automatically placed against the discs and when the driver finally does use the brakes, the system monitors the pedal pressure. If the pressure is determined to be too light, the braking power is amplified by the system.
It's a great idea, but the TV commercials for this system need some serious attention. Volvo's commercials actually show a woman driving a Volvo, arranging papers on her passenger seat and talking on a cellphone. When the collision warning system activates and she looks up, bemused, then applies the brakes to avoid running into a truck in front of her - a truck that she would have seen and presumably slowed down for had she been paying attention. I know it's not meant to be taken this way, but that Volvo commercial actually appears to be promoting distracted driving - Volvo will attempt to save you from your own ineptitude because apparently it's just too inconvenient now to be paying attention to the road ahead.
Rather than train drivers to understand that they need to be responsible for their actions, that they need to be alert to their surroundings and that they need to pay attention when they're driving, collision warning systems essentially attempt to treat the symptoms rather than trying to cure the problem itself.
Brake-assist and auto-brakes go one step further. In some high end vehicle now (top end BMWs and Mercedes' for example), the collision-detection system is linked into the brakes like it is with the Volvo system, but it's also been given the flexibility to do all the braking for you. Adaptive cruise control, for example, will control the throttle just like a normal cruise control system, but will also apply the brakes if it determines that you're getting too close to the vehicle in front. Full auto-brakes will actually stop the car for you if you fail to respond. All these systems work in essentially the same way - they monitor the brake use and distance to the vehicle in front. If the computer thinks you're not braking hard enough, it will assist you.
These systems are all very clever but they tread the thin ethical line. Just because engineers can make their vehicles do this doesn't mean they should. Consider this: with in-vehicle monitoring and tracking systems like OnStar, and the impending satellite-tracking systems for road tolling, it's not too hard to imagine all those systems chained together in such a way that the vehicle will literally prevent you from speeding by limiting the throttle availability and controlling the brakes. If you really want to be driven like that in a vehicle over which you have no control at all, take the bus.

Now don't misunderstand me here - I think a lot of what Volvo do in vehicle safety is a good idea - the transparent A-pillars, the blind-spot assist and things like that - they all go towards eliminating problems inherent with the design of cars. But I believe putting systems into a car that attempt to compensate for the ineptitude of the person behind the wheel is a mistake. But that's just my opinion.

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Other Brake Technologies

There are other brake technologies that are becoming available in vehicles now, and a lot of them are gathered together in the 2006 / 2007 BMW models. They're the rolling embodiment of clever brake engineers just showing off. Three of the more notable features are:

All the above devices fall into that ethical grey area again, but unlike the brake-assist and collision-detection systems outlined earlier, these three brake technologies don't actually attempt to compensate for any wrongdoing on the driver's behalf. They simply help prepare the car for when the driver chooses to use the brakes. From that point of view, I would regard these as better technologies than those which go the whole hog and interfere with your driving.

Brake hoses - not just rubber.

Obviously with all the pressure in your brake system, the last thing you need is for the brake lines themselves to deform and flex. If they do, you lose brake pressure, and thus lose braking. Steel brake lines are no problem, but for the flexible areas of the brake lines, you need hoses. Brake hoses come in two basic flavours.

Rubber hoses.

A rubber brake hose

Ah the humble rubber hose. Only on your brake lines, not so humble. I don't recommend this but if you were to get under your car and cut one of your hoses in half, you'd notice a couple of things. First, it's amazing how quick all the brake fluid that spills out will stain your clothes and literally eat the paint off your car right in front of you. But second, and more importantly, the hose itself is actually made of three parts. The inner liner is a corrosion and brake-fluid resistant compound designed (normally PTFE / Teflon® based) purely to keep the brake fluid in. Around the outside of that, there's a steel webbed mesh. This is what gives the brake hose its strength and stops it from bulging and deforming. And around the outside of that there's a slightly thicker rubber coating, which is there to weatherproof the steel mesh. The three layers together give strength, flexibility and durability.

Steel-braided hoses.

A steel-braided brake hose

Steel-braided hoses are a slightly different design. They only really have two components - the inner hose which carries the brake fluid and is lined with a PTFE compound, and the outer steel braid which contains and flexing or bulging. Steel-braided lines resist bulging a lot better which is why a lot of aftermarket tuners opt to put them on their vehicles. One downside is that the steel braiding itself is totally merciless and if it finds something to rub against in the vehicle, it will rub right through it, even if it's an alloy. For that reason, a lot of braided brake hose manufacturers put a third layer - a thin transparent rubber sheath around the outside simply to keep everything in check and prevent scuffing and rubbing.
I upgraded the lines on my Audi when I still owned it and put Goodridge steel braided hoses on. For a 15 year old car it did make a difference to the feel of the brake pedal. It didn't bring it up to modern standards, but it was better than the flexible, bendy rubber hoses that were on it from the factory.

Brake fluids.

havoline brake fluid

As mentioned elsewhere on the page, brake fluid does not compress. It's a good job too - if you put your foot on the brake pedal and it went all the way to the floor, you'd be worried. But that's exactly what can happen if you disregard the "health" of your brake fluid.
Brake fluid is hygroscopic - that means it attracts and soaks up water. This is why it comes in sealed containers when you buy it, and why when the crazy guy four doors down offers you some of the 15 gallons of brake fluid he's had in his garage since the war, you should turn him down. The problem with it being hygroscopic is that if it does start to take on water, Bad Things can happen. Pull up a chair and allow me to explain.
Your typical DOT 4 brake fluid (see later for DOT ratings) boils at about 446°F (230°C). Water boils at 212°F (100°C). Imagine your brakes are getting hot because of a long downhill stretch. Whilst the brake fluid is quite OK, the temperature of the brake components might get up over the boiling point of water. If that happens, the water boils out of the brake fluid and forms steam - a compressible gas. Next time you put your foot on the brake, rather than braking, all the pressure in the brake system is taken up with compressing the steam. Your brakes go out, you don't stop.
Getting a little more complex, the boiling point of a liquid goes up with its pressure (Physics 101). So when you step on the brake, the boiling point of the brake fluid might actually go up to 500°F (260°C) and the boiling point of the water content might raise up to 250°F (121°C). This is great, you might think, because now the boiling point is higher than the temperature of the brake fluid. At least it is until you take your foot off the brake again. Now the pressure in the system returns to normal, the boiling points revert to normal and instantly the water boils off into steam again. The symptoms are slightly different now. Under this scenario, the brakes work the first one or two times, but on the third or fourth press, they stop working because now the temperature and pressures have conspired to boil the water.
The worst possible scenario is brake-fade (see right at the top) combined with air in the system. If this has happened to you, then you're likely reading this page from beyond the grave, because in most accidents where weak brakes become no brakes, there aren't any survivors.

D.O.T ratings

All brake fluids are DOT rated. Your owners handbook for your car or motorbike probably tells you to use DOT3 or DOT4 from a sealed container. The DOT ratings are a set of minimum standards the fluid must adhere to in order to get the rating, and thus work in your braking system. The following table shows the various properties of DOT ratings. Remember that the values here are the minimum values. Most manufacturers make sure their product far exceeds minimum ratings.

Boiling Point DOT 3 DOT 4 DOT 5 (silicone-based) DOT 5.1 (non-silicone based)
Dry 401°F 446°F 500°F 500°F
Wet 284°F 311°F 365°F 365°F

The "dry" and "wet" boiling points in the table above are for brake fluid which is fresh from the bottle (dry) and which has a 10% water content (wet). A DOT study in 2000 discovered that on average, the brake fluid in a vehicle absorbs about 2% water every 12 months.
The two types of brake fluids shown in the table are DOT3/DOT4/DOT5.1 which are glycol (Polyalkylene Glycol Ether) based, and DOT5 which is silicone based. DOT3 and DOT4 fluids are interchangeable* - the only real difference is their boiling point. Theoretically you could interchange DOT4 and DOT5.1 fluids too but I wouldn't recommend it. DOT3/4/5.1 and DOT5 fluids cannot be mixed or interchanged under any circumstances. They mix like oil and water (ie. they don't) and the silicon based fluids can destroy the seals in brake systems which rely on the moisturiser additives that are present in DOT3/4/5.1 fluids.
Other things you ought to know about silicone based fluids:
- they are resistant to absorbing water, which is why their wet boiling points are so high. Problem is that any water content eventually pools in the low spots of the brake system and causes rust.
- they don't strip paint.
- they are not compatible with most ABS system because they doesn't lubricate the ABS pump like a glycol based fluid.
- putting this fluid in systems which have had DOT3/4 fluid in will cause the seals in the caliper and master cylinders to malfunction. Which means they need replacing. Which is expensive.

Oh, and don't ask me why DOT5.1 is glycol and DOT5 is silicon based. It doesn't make and sense to me either.

* There has been some discussion as to the use of DOT4 fluid in Toyotas that recommend DOT3 fluid - apparently something in the Toyota braking system doesn't play well with DOT4 fluid, particularly the master brake cylinder seals. The discussion about this can be found in the archives at the UK Pruis yahoo group.

Brake warning lights

brake warning light

Most cars nowadays have a brake warning light on the dash. Its purpose is to alert you that something is wrong in the braking system somewhere. If it comes on, check your owner's manual to find out its meaning. Unlike the single-purpose ABS warning light, the brake warning light doesn't have a standard meaning; it could be used for multiple purposes. For example, the same light may be used to show that the hand brake (parking brake for the Americans amongst you) is on. If that's the case and you're driving, you ought to have noticed the smell of burning brake dust by now. The light can also indicate that the fluid in the master cylinder is low. Each manufacturer has a different use and standard for this light. Which is nice. Because it would be such a drag if the same indicator meant the same thing in every vehicle.


brake warning light

If you've got an ABS-equipped car, you also have a second light - the ABS light. If it comes on, get it seen to as soon as possible. It means the ABS computer has diagnosed that something is amiss in the system. It could be something as simple as dirt in one of the sensors, or something as costly as an entire ABS unit replacement. Either way, if that light is on, then you, my friend, have got 1970's brakes. It's important to note that this light normally comes on when you start the car and then switches off a few seconds later. If it stays on, blinks, throbs, flashes or in any other way draws your attention to itself, take note. It's not doing it just to please itself.


brake warning light

If you see this light on your dashboard, then congratulations - you're flying the service module on an Apollo mission. The bad news is that you've got a current drain somewhere and your main batteries are critically low. Either way, drop me a line and let me know how you snagged a seat on a spaceflight - I'm dying to know.


And finally....LED replacement bulbs

LED brake bulbcycoactive LED replacement bulb

You might have seen websites and automotive shops stocking LED replacement bulbs for cars and motorbikes. The most basic replacements look like those on the right - a cluster of 19 or 24 LEDS (light emitting diodes) in a housing with a regular push-fit or bayonet plug on the back. The idea is that if you want brighter lights, you can replace your tail or turn lights with these LED replacements. There is a gotcha though that the manufacturers often hide in the smallest of small print. A lot of vehicles (cars and motorbikes) have onboard diagnostics which include a light check. Some of these use resistance to figure out if a bulb has blown. LED clusters have a radically different resistance to a filament bulb and its possible that when you replace your bulbs with LED versions, your car will continuously tell you that one or more bulbs is burned out. Getting one step more severe, if you use LED turn bulbs, your indicators could flash quicker or slower than you're used to (indicator circuits use the natural resistance of the bulbs in conjunction with the relay to dictate the flash speed). The worst case scenario though is ABS; some motorbikes have very tightly regulated voltages in their ABS systems and taking the filament bulb out of the brake light to replace it with an LED bulb can cause ABS errors and theoretically, an ABS shutdown. Granted thats worst-case scenario, but it is a possibility.
The way around all these electrical load problems is to add resistance or ballast to the bulb replacement, and this is becoming more common now. Essentially, resistors are added in-line with the LEDs to provide the same sort of resistance to current flow that an incandescent bulb would, thus making the retrofit kits far more compatible with existing car or motorbike electrical systems. CycoActive make kits like this for motorbikes now. You can see an example on the left here which shows the LED unit as a complete replacement for the tail unit on a bike, with a bayonet connector to fit into the old socket. The blue items are the ballast resistors designed to induce sufficient load in the electrical system for the diagnostics to register the unit as a regular lightbulb.

Picture credit: CycoActive