The Sinking Brake Pedal

Apr 25, 2015

RAY: Here's an automotive puzzler from Bob Gossam. He says, "Some time ago, I married a woman with a Datsun B210 -- The Honeybee, it was called. It was small, noisy, reliable, and fun.

"But, one day, I had a problem. I pushed on the brake pedal, and it went almost to the floor. I didn't panic. I just pumped the brake pedal a little, and it came right back.

"I had the brakes checked out stem to stern, but there were no leaks, and there were no problems with the master cylinder. The calipers in the front were fine. The wheel cylinders in the back were okay.

"Still, the problem persisted. After much observation, I determined that it only happened after the car had been driven above 40 miles an hour for 10 minutes or more. After that, if I hit the pedal again, it would be okay, unless I continued to drive on the freeway, in which case it would recur at 10-minute intervals.

"I took it to garage after garage... and no one had any ideas what the problem might be.

"One day, I'm having a beer with an engineer friend of mine, and I tell him about this problem, and of course all the gory details. He asked me one question, and then told me what was wrong."

TOM: Is the question, "Is the seat bolted to the floor?"

RAY: That would be something you would ask, given the junkers you drive, wouldn't it? No. The question he asked is, "Did you buy something recently for this car?"

And the answer was, "Yes I did."

So...what DID he buy?

RAY: What he bought was a new tire, and when they installed the tire on the car, they didn't balance it correctly, or didn't balance it, perhaps, at all. And as the car is going down the road and the thing is shaking, it is setting up a sympathetic vibration in the disc rotor, which is moving the caliper piston away from the disc, so that when you step on the brake that first time, the pedal will travel almost to the floor. We've seen this happen with cars whose wheel bearings are loose.

TOM: I actually like this answer.

RAY: And when you step on it that first time, that caliper has to move. Because the caliper piston is not touching the brake pad. And then finally, when it does make contact, the second pump is OK. And of course, you drive along for 10 more minutes, the vibration occurs. And he said it was undetectable, virtually, because the car had so many other vibrations and moans and groans or whatever, he never noticed it. And the hint was that it was a Datsun B210. Anyway, who's our winner, Tommy?

TOM: The winner is Erica Coffin from Austin, Texas. Congratulations!

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