This phenomenon known as brake fade can be quite lethal and unexpected and i also would hazard a guess that 1 in 4 cars may very well experience brake fade to some degree.
Brake fade manifests itself from a spongy feel on the pedal and/or virtually no braking force. Do not confuse brake fade with bad brakes or perhaps a mechanical fault as it pertains to brake systems in relatively good condition.
You might be at greater risk from brake fade unless you maintain your brakes pads, discs and properly should be regularly checked for signs of wear, and discs should be periodically roughed up if they appear smooth and glassy. One issue often confused with brake fade actually causes a spongy pedal feel.
When you use your brakes you might be converting the forward momentum of the car into heat. The heat will build up in the discs where it will be dissipated into the air. If water is present in the brake fluid you are very likely to experience brake fade since the water heats up and boils, the pads will also get warm and also this heat will also transfer over the pistons and callipers in the brake fluid and. The heat can also increase the risk for brake hoses to expand a little reducing the hydraulic pressure further.
The spongy pedal feel is down to either air getting into the brake line (improper bleed and refill or a leak) or, and much more commonly, by water stepping into the brake fluid. Because the brake fluid is Hygroscopic it draws in water which has totally different characteristics to brake fluid when it comes to boiling points. This type of water within the brake fluid boils and releases gases (steam) causing spongy braking as unlike fluids gases are compressible. This problem is referred to as vapour lock where fluid has reached its boiling point which is determined by the portion of water inside the brake fluid. A dot 5.1 brake fluid performs better and over a prolonged period of time without the issue of water retention.
All cars are susceptible to brake fade but driver habits can be a major cause, along with poor maintenance!
The primary reason for brake fade in a good well maintained car is the heat build up which may adversely modify the friction as well as cause warping or distortion of the brake components. Brake fade when this happens willusually and still, use a firm pedal but the conversion of momentum into heat cannot effectively happen hence the car is not going to slow up.
A brake pad are going to smoke as it gets beyond its operating temperature, the layer of gas emitted will behave as a lubricant between the two friction surfaces and will be manifest as ineffective brakes (This is known as out gassing). The effectiveness of the friction surfaces also reduces as heat increases which is why performance brakes are designed to operate at the much higher temperature than OEM parts. (The most obvious downside of performance brakes is poor cold braking, not something you want on a road car.)
The first tip is somewhat obvious but I still see so many people doing this one. To avoid brake fade do not keep your brakes on when driving down a hill, change into a lower gear and let the engine have a safe speed for the car.
When people arrive at the bottom of any hill they suddenly realise that their brakes are will no longer effective. (If the brakes get really warm you also run the risk of binding where the pads get welded to the brake disc)
As opposed to brake heavily at the very last minute try to spread your braking over a longer distance. An emergency stop from 70 might be enough to result in brake fade and these higher speed speeds a failure of the brakes can be quite lethal. So, jab the brakes quite hard initially and kill most of your forward momentum, the pace you are travelling at will assist the discs to dissipate the high temperature and then reapply the brakes more gently bringing the automobile to a stop. If you have made heavy utilization of your brakes always think that fade will manifest itself for the next few minutes and increase your distance from the car behind whilst keeping your speed down a bit.
On the track is not uncommon for the heat in the brakes to cause the discs to glow red or white hot and cheap pads have already been known to smoke, a good quality pad for the track is designed to cope with the high temperatures and often not 100% effective until it warms up. When you have new brake pads or discs fitted you should use them gently at the beginning as they will not be effective until they are properly bedded in.
Use a dot 5.1 brake change and fluid the brake fluid annually. Larger discs and high performance pads will also help minimise brake fade although at the extreme end they may exhibit lousy cold braking properties. See our article on brake uprating for several tips and ideas on uprating your brakes and further reduce your probability of experiencing brake fade.