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Author: ptheland Big gold star, 5000 posts Feste Award Nominee! Old School Fool Add to my Favorite Fools Ignore this person (you won't see their posts anymore) Number: of 11691  
Subject: Re: UAI - Kia Sorrento Date: 9/4/2012 2:07 PM
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Now I'm not going to further knock anybody who gets it wrong, but will say that I don't understand it.

There are a lot of people out there who don't really understand how their car works. They just know you press on this pedal to make it go, and that one to make it stop.

For those people, if the brake doesn't work the way they're used to it working (which is often overboosted and only taking a light touch to get a lot of stop), they'll let up and try again.

And in a different "failed brake" situation - say a leaking master cylinder - pumping the brakes is the right thing to do.

Maybe it only seems like common sense to me because of my knowledge of how the system works?

Probably.

If the brakes do work on the first press, why would one think they have failed and begin pumping the pedal?

Remember that your typical driver only knows their brakes when the brakes aren't fighting against the engine. They're generally taught to use only one pedal at a time. Brake or throttle - not both. Mixing a little brake with some throttle mid-corner to get the car to settle down is not something grandma is going to be doing on a regular basis. ;-)

So when the throttle sticks and they apply the brakes, the car isn't stopping the way they're used to it stopping. It doesn't surprise me that they call it a "brake failure" even though the brakes are working normally. Then some old bit of driver's ed or urban legend kicks in and they start pumping the brakes. There goes the vacuum reserve and now they have un-boosted brakes.

An un-boosted brake pedal is rock hard, but with little braking power. It takes all the muscle you've got to have some effect on the car. Depending on the power of the engine and the strength of the leg, you may not be physically able to slow the car at that point. And yet there is still nothing wrong with the braking system. If the throttle gets un-stuck and the car is inspected, you'll find nothing wrong with the brakes. And yet the driver will insist that the brakes failed. More correctly, the brakes had less effect than the driver is used to, first because of the stuck throttle and then because of a depleted vacuum reserve.

Peter, I'm curious.... In your testing, was the first (boosted) attempt at braking your car effective at slowing it despite the full throttle?

Somewhat. It was definitely less effective that normal.

What about successive (unboosted) attempts?

Much less effect. Since I didn't really have a stuck throttle and had to keep one foot on the accelerator, I could only use one foot on the brake. I couldn't actually slow the car with just one foot. If I had both feet, I think I could have accomplished some slowing of the car. But it definitely took everything I had to give.

What make/model is your car?

Do I really have to answer that? OK. 1999 Honda Odyssey. And a similar effect in the 2001 Chrysler Town & Country. The T&C had a bit more braking - partly because the engine has a few less horses, and partly because it has disc brakes on all four corners. The Honda has drums in the rear.

I'd encourage you to give it a try for yourself. Find an open bit of road, floor the throttle, and see what the brake does, both on the first press and again after a couple of pumps.

--Peter
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