Did Julia Cause Brake Failure?

Dear Car Talk

Dear Car Talk | Mar 03, 2015

Dear Car Talk:

Is it possible to drive with a parking brake on? I was driving, and then all of a sudden my brakes failed (I was going about 50 mph). I swerved off the road and went into a ditch. If I had accidentally left the parking brake on, would it have messed up the normal brakes? Someone told me that's what happened. It was in an older Honda Civic (about 10 years old, maybe).

-- Julia

Yes, it is possible to drive with the parking brake on. It happens all the time. Lots of people apply the parking brake lightly when they park. And unless you really pull (or push, if it's a foot-operated brake) the parking brake until it almost won't move any more, the engine can overcome it and move the wheels.

When that happens, you might notice that the car seems a little sluggish. Sometimes people will notice an odd burning smell after driving for a while. More observant drivers will notice the big red light that says "BRAKE" lit up on the dashboard. But you'd be surprised how many people don't notice anything until they go to park again and say, "Hey, that brake's already on!"

Here's how it makes the brakes fail: When the parking brake is on, even a little bit, it's as if you're driving with your foot on the brake pedal -- also known as riding the brakes. When the brakes are applied, the friction of the pads against the rotors produces heat. And when you leave the brakes on for a long time, especially if you're driving at 50 mph, that produces a lot of heat. Eventually, the heat will get transferred to the brake fluid, and the brake fluid will boil.

Boiling brake fluid can't transmit pressure to the brakes. So you step on the pedal, and you relieve yourself. And drive into a ditch. And if you're lucky, you live to write to Car Talk and ask what the heck happened.

So that's an entirely plausible scenario, Julia. Did you notice that the parking brake was partially applied when your heart returned to fewer than 400 beats per minute?

If so, then that's almost certainly what happened. In which case I'd ask your mechanic to take a look at the brakes to make sure you didn't overheat them to the point where you did some damage. When you drive with the parking brake even partially on for several miles, it's possible to warp a drum or disc. Or if the brakes get really overheated, you can even cause the lining's adhesive to fail, and have the linings crack or even separate from the pads or the brake shoes. And that would need to be fixed.

But if nothing is actually damaged, then all you need to do is remember to disengage your parking brake before driving. And I have a feeling you're going to remember that from now on.

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