In a recent column you addressed a brake problem on...

Dear Car Talk

Dear Car Talk | Apr 01, 1994

Dear Tom and Ray:

In a recent column, you addressed a brake problem on a 1991 Toyota Camry. The symptoms brought to mind a similar problem with my father's 1991 Toyota pick up truck. I drove the truck on two occasions, and experienced a bad shimmy after driving about five miles. There was also an excessive amount of brake dust on the wheels. The truck was serviced three times by the dealer, who could not identify the problem. The calipers and rotors were changed twice, and the last item replaced was the master cylinder. My father had an accident in the truck and was fatally injured. I suspected the brakes had failed, and had a consultant inspect the truck to find the cause. It turned out that an adjustment on the master cylinder was improperly done, and the brakes were not fully releasing. I suspect that my father experienced boiling of the brake fluid and could not stop at a red light. He drove more slowly than I, and had driven a considerable distance before the accident. Please make people aware of the danger of fluid boiling when brakes drag.

RAY: We've mentioned it many times, PC. When the brakes get stuck "on" for some reason, the heat from the friction can make the brake fluid boil, which more brakes!

TOM: In your father's case, I'm not sure the master cylinder was responsible. In our experience, it's been the power brake booster that causes this problem. If the booster is faulty or the pedal rod is misadjusted, the booster can stay on, and make the brakes act as if the driver's foot were pushing lightly on the pedal. That would explain the excess dust, the shimmy (disc warping due to heat), and ultimately, the brake failure.

RAY: That's why extensive road testing is necessary whenever you're dealing with a brake problem. It's possible that if the dealer had driven it for a long time, and then put it back up on the lift, he would have noticed that the brakes were locked up.

TOM: And by the way, for you summer vacationers, severe OVERUSE of the brakes can also cause brake fluid to boil. That's why it's important to downshift, and use the engine to keep the car at a safe speed when you're descending a long, steep hill. If you keep your foot on the brakes for miles and miles as you wind down a mountain side, you could heat up the fluid and lose your brakes there, too.

RAY: Not the best way to descend Pike's Peak...although at least you wouldn't be stuck behind all those RVs.

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