Easy way to ruin brakes

First Posted: 3/4/2015

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.

Dear Car Talk:

I have a 2012 Kia Forte. It’s a great car, except that in the winter when I turn on the defroster, the engine runs really rough. As soon as I turn off the defroster and turn the switch to “heat,” the engine runs smoothly again. I want to know if you have heard anything about this, or is this normal? — Shirley

It could be normal, Shirley. If the engine runs just slightly rougher when the defroster is turned on, that could simply be because the defroster uses the air conditioner.

Because the air conditioner removes moisture from the air (i.e., it conditions the air), when you turn on the defroster in most cars, the air conditioner is automatically switched on, so the air blowing on the inside of the windshield is dry air. And because the air conditioner imposes an additional load on the engine (it takes a lot of engine power to run), some cars with small engines will seem to run a little rougher when the AC is on.

But it should be barely noticeable. So test this theory yourself by turning on the air conditioner without using the defroster. If the car runs smoothly, then it’s not normal, and I’m guessing you have a vacuum leak.

There are little “doors” in the ventilation ducts that open and close to direct air from one place to another. They’re called blend doors. They’re usually operated by vacuum motors. And if the vacuum motor that directs air to the windshield has a leak of some kind, that will make the engine run rough when you engage that setting.

The reason it makes the engine run rough is because the vacuum is generated by the pistons when they go up and down. If there’s a leak in a vacuum hose, it allows extra air to get sucked into the combustion chambers, and throws off the carefully calibrated fuel-to-air ratio.

The bad news is that in order to get at this blend door, your dealer may have to remove the dashboard. The good news is that Kias have a five-year, bumper-to-bumper warranty. And last time I checked, the dashboard was still between the bumpers.

So take the car to your dealer and show him what’s happening, and ask him to fix it. Good luck, Shirley.

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