Heat degrades brake performance 2 different ways. Most common is over heating the lining enough for liquid to ooze out to the surface lubricating the lining. Given a chance to cool, it may form a glaze reducing braking even when cool. Sometimes the glaze can be sanded off, but the brakes may not really work right until the abused lining is replaced. Your metallic and semimetalics are more resistant to this.
Extremely hard braking may heat things up to where enough heat is transferred to the fluid to raise the vapor pressure above atmospheric pressure. When you release the pedal and the pressure drops, vapor forms displacing the fluid back to the master cylinder. Next time you apply the brakes, the vapor compresses, maybe even reliquefying. However you run out of pedal before forcing enough fluid into the line to build any pressure. Water, brake fluid, and any combination of them are liquids, and never compress. The more water in the fluid, the lower temperature it takes to produce the compressible, brake destroying vapor. Since it is very difficult it keep all the water out of a real world system, brake fluids are designed to maintain a high boiling point even when contaminated by some water. Any air in the system causes the same problems. Other components flex enough to create somewhat of a soft pedal. Lines stretch, the flexible rubber ones the most, calipers and brake drums flex. The firewall gives a little.
I worked in a brake shop specializing in older trucks for 3 years. The old coot that owned it had been there over 40 years. I never heard anybody mention changing brake fluid. I did hear a lot about bleeder screws with a broken Easy Out in them. He also strongly believed in rebuilding calipers and wheel cylinders every lining change. Rubber against steel wears away. It may even lose its elasticity eventually. If you want to play mechanic and change the brake fluid, go ahead. Just don't try to tell anybody it is necessary or likely to solve more problems than it causes. Leave well enough alone, especially if you don't know another way to handle a stuck bleed screw than pulling harder.
I have never changed the brake fluid in my 77 truck or the 3 cars I traded off at 10 years or more, and I have never had to replace any corroded hydrolic brake parts. I do rebuild the cylinders and calipers now and then.
[ December 04, 2003, 10:28 AM: Message edited by: labman ]