Any idea what could cause a sudden brake failure?

Dear Car Talk

Dear Car Talk | Aug 01, 1998

Dear Tom and Ray:

My adult daughter was driving my 1988 Toyota Camry when she was involved in an
accident due to reported failure of the brakes. She badly bruised the palm and
little finger of her right hand in an attempt to apply the emergency brake. It
was a hot and humid day. The Camry, with about 91,000 miles on it, has been well
maintained. The Toyota dealership, to which the car was towed, could find no
evidence of brake failure. Do you have any idea what happened? -- Mary

RAY: I have a couple of ideas, Mary. One is that your daughter was driving with
the parking brake on. Even if it was only applied partway, that could still be
enough to heat up the brakes and cause a problem.

TOM: When a brake is left on like that, the heat from the resulting friction can
actually make the brake fluid boil. And boiling brake fluid can't make the car

RAY: That would also explain why the dealership found nothing the next day. Once
the parking brake was released and the brake fluid cooled off, there would be no
evidence whatsoever. In fact, even an hour later there wouldn't be any evidence.

TOM: If your daughter is absolutely sure she didn't leave the parking brake
partway on (if she's one of those people who has already made this bone-headed
mistake once and now checks the lever twice before putting the car in gear),
then there are a couple of other possibilities. One is that she's a "two-footed-
driver," which could also explain overheated brakes. You can test that theory by
driving behind her and seeing if the brake lights are constantly flickering on
and off.

RAY: It's also possible that your daughter has nothing to do with it. A sticking
brake caliper would have the same effect as leaving the parking brake on. The
only reason I'm less inclined to believe that is that the dealership would
probably find evidence of a sticky caliper after the fact.

TOM: Another possibility is a master cylinder failure. That could also fail
intermittently (and, under rare circumstances, show no evidence later), but I'd
lean toward the parking-brake theory.

RAY: And by the way, Mary, you probably have a pretty good idea now why car
makers -- at their lawyers' suggestion -- have changed the name from "emergency
brake" to "parking brake."

* * *

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