I was once asked for information about the potential for temporary self-correcting brake failure. The answer: yes it can happen. But only in very unusual circumstances.
The description provided by the driver typically goes something like this: after driving a long stretch on the highway, they reached for the brake pedal and it went to the floor. They pumped and pumped to no avail. An inspection a few hours (or more) later showed the brakes to be in perfect working order.
Brake System Background
Most brake systems are split into two circuits nowadays (some fancy and complicated schemes overlap the two circuits, making effectively three or four, but they are rare, so I'll ignore them.) Thus if a failure develops in one section of the brake system, the other circuit should (hopefully) provide some braking capacity. Some vehicles have diagonal-split systems, linking the front left brake to the right rear brake. Other vehicles have a front-to-rear split, linking the front brakes together in one circuit and both rear brakes in another.
Typical DOT-3 brake fluid normally boils at about 220 degrees C (DOT-4 fluid is higher, and the synthetic DOT-5 fluid is higher still). As DOT-3 or DOT-4 fluid ages, it absorbs water (being hygroscopic) through the hoses, cylinder boots and seals, etc. After just a couple of years, the fluid may have absorbed several percent worth of water, lowering the boiling point to 150 degrees C or less. Fluid will even absorb water through the walls of a closed plastic bottle on the shelf, so one should avoid using even unopened old fluid. 150 degrees C is not that uncommon in passenger vehicle brake components, but generally the heat dissipates before it can boil the fluid. (DOT-5 fluid has a slightly different problem: since it isn't hygroscopic, it doesn't absorb water. The water that seeps in through the hoses forms puddles of 100 degrees C boiling point fluid in the line.)
While the brakes are not applied, there is a compensating port in the master cylinder which allows fluid to drift lazily back and forth to the reservoir. This helps ensure that the calipers and cylinders actually release when you lift off the pedal. If the fluid in one circuit heats to the boiling point, it becomes gas (taking up much more space), which pushes cooler fluid further down the line towards the reservoir, potentially overfilling it. When you step on the brake pedal, before you get any motion at the wheel cylinders or caliper pistons, you have to pump that fluid back into the lines. Once the system cools down, the fluid from the reservoir which was pushed out should be partially drawn back into the line through the compensating port, but there'll probably be some partial vacuum left, and the reservoir level may be elevated above its normal height. The process of refilling the lines with fluid from the reservoir will be completed during the first pump (or two) of the pedal, with the reservoir level falling slightly to the pre-accident height. This change in fluid depth may not be enough to clearly measure. After this action, all will be well again.
The Potential Problem
For complete brake failure, both brake circuits have to get hot enough to boil the fluid. Degraded fluid makes that happen sooner. The most likely case, as I see it, is someone driving a long way with their parking brake on a bit, overheating both drums in a diagonal split system. In a vehicle which has a front/rear split, this same scenario may cause the rear brakes to be useless, and the pedal to feel awful (maybe going to the floor), but the front brakes will still work.
There are numerous reasons for brakes to drag either singly, in axle pairs, in circuit pairs, or all at once, including a booster which fails to release fully, caliper(s) with sticky slides, a malfunctioning combination valve, internal hose damage, and mechanical hangups in the parking brake system. These alternative possibilities, however, don't generally have the same potential for affecting both circuits at the same time in such an extreme fashion as the non-released parking brake in a diagonal split system.
The amount of heat required to boil good fluid will leave clear evidence: blued steel (or iron), and glazed, cracked, checked, or badly degraded friction material which smells of overheating. Nearby combustible bits may be set alight. Degraded fluid will reduce the amount of overheating required, so it may reduce the post-accident evidence, also.
Investigation
Careful and thoughtful observation of the first couple of pedal applications after this type of occurrence is critical to diagnosis. If an investigator suspects that this has occurred, he/she should consider notifying any other concerned parties of the impending test to allow them opportunity to watch/participate, to prevent spoilation of evidence questions later. These first pedal applications are destructive in nature, and cannot be repeated later.
If the brake pedal has been depressed since the incident, the only remaining evidence will likely be the condition of the rear brakes, and the consistency of the witness statements with this phenomenon. If no mechanical problem can be found which would contribute to the rear shoes' dragging inside the drums, but there's evidence that they did, one must consider the possibility that the brake was not fully released, or that the operator drove with one foot lightly on the brake pedal, dragging the brakes enough to boil the fluid in that way.
*Wade Bartlett, BS, MS, MSAE, is the principal of Mechanical Forensics Engineering Services, a US company specialising in accident reconstruction. This story is current at September, 1997.
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Footnote: the vehicle accidents shown here are for illustrative purposes only; they did not necessarily result from brake failure.
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