Here are a couple of mysteries for you; or rather, two versions of one mystery.
Scene one: A race car is taken out of storage for its first race of the season. The driver starts out running several slow laps to get everything up to temperature, then starts driving in earnest. Half way through his second fast lap, he brakes for a chicane and the brake pedal suddenly goes to the floor -- no brakes. The car hits the tire barriers, damaging the bodywork and ending practice for the day.
Then comes the strange part: While the driver is sitting in the car, waiting for the course marshals to remove the tires and other things that landed on the car, he pumps the brake pedal and it returns to normal. He has brakes again and the car can be driven back to the pits as if nothing happened.
Scene two: A different driver went for an afternoon drive from Derry to Milford and back, via two different routes. On the way out, he went via Route 101, which is mostly highway. When he got to his destination, he noticed that one of the rear brakes was sticking -- the brake caliper was not disengaging completely and the brake pads were dragging on the disc. As a result, the brake and the wheel were both too hot to touch. The driver paid it no mind. On the way back, he followed Route 101A through Nashua, over the river to Hudson, then up Route 102 (Nashua Road) home; total distance, 51.5 miles.
Driving north from Hudson, he came to the first traffic light in Londonderry, stepped on the brakes and...Nothing. The pedal went to the floor. He downshifted the transmission to slow the car and rolled to a stop. He continued to slow the car with the transmission for the next five miles and several traffic, then discovered the brakes were working again. The last few miles of the trip were completed without incident.
Now the mystery: What did these two events have in common and why did both cars regain their brake power after losing it?
The answer: A combination of heat, old brake fluid and water.
Here's what happened.
Most types of brake fluid are hygroscopic: that is, they absorb water from the atmosphere. Since water is heavier than brake fluid, it eventually settles through the brake master cylinder (the pressure pump part connected to the brake pedal), through the brake lines to the brake calipers. Once there, it acted like brake fluid, which is incompressible stuff and transmits hydraulic force. Or it does until the brakes heat up under hard use as in the race car, or if a brake is dragging and constantly causing friction.
When the brake caliper and the fluid inside heat up over the boiling point of water, the water in the caliper turns to steam, which is a vapor. By its nature, steam is compressible. Now, instead of the brake pedal pushing against fluid, it is pushing against steam bubbles that are big enough to take up space from the fluid in the master cylinder. Thus, the brakes are disabled even though they are still in good mechanical condition. When the brakes cool below 212 degrees, Fahrenheit, the steam turns back into water and the brakes magically work again.
So what lessons can be learned here?
First, keep your brakes in good working condition. The dragging brake described above was the result of a sticking caliper that should have been replaced at the first sign of trouble, rather than ignored because "nothing happened yet." Deferred maintenance is an invitation to disaster, be it a dragging brake, a cam timing belt that is beyond its replacement date or any other part that wears out over time.
Second, consider having the brake system flushed and refilled with new fluid periodically. This should be done when the brakes are bled after a brake job -- new pads, new or resurfaced discs, etc. If your car sits idle for long periods (as the race car did), the fluid should be changed as a precaution. Ask your mechanic for his advice on how often the fluid should be checked.
Third, get to know your car. Learn how you can slow it down without using the brakes and how much extra distance to allow if you do have brake problems. And always leave yourself enough distance to stop safely, no matter how good your brakes are.
One final note:
The incidents described above are exceptional cases -- the brake discs and calipers were considerably hotter than they would get in normal street driving, so don't panic. You're not likely to run into such circumstances if your car is in good condition and driven normally. Keep your car -- and its brakes -- in good working order and you drive worry free for a good long time.