Should I be concerned about my brake pedal slippage?

Dear Car Talk

Dear Car Talk | Jan 01, 1998

Dear Tom and Ray:

I have a '94 Dodge Shadow. When I stop at a red light and I have my foot on the
brakes, I can feel the brake pedal slip and go down a little. I called my
mechanic, and he said it could be the master cylinder and told me to take it
back to the dealer. The dealer checked it and told me that nothing's wrong.
They didn't charge me for the checkup. What do you think? Should I be
concerned? -- Ellen

TOM: Well, I'm concerned that they didn't charge you. We're going to have to
report this dealer down at the Club. He's making the rest of us look bad.

RAY: It could still be a bad master cylinder, Ellen. Leaky master cylinders are
notoriously difficult to diagnose.

TOM: But it could also be absolutely nothing. We see this on a lot on modern
cars with electronic engine-management systems (and for the sake of this
discussion, we'll consider a '94 Shadow a "modern car"). When you come to a
stop, the engine-management system may increase or decrease the idle speed
slightly for a variety of reasons. When that happens, the engine vacuum changes
and, since the power brakes are vacuum-assisted, you could feel a slight
sinking of the brake pedal.

RAY: The problem is, how do you know whether it's normal or not? The answer is
that you don't. But your mechanic should be able to tell. And if he's unsure
about it, I would suggest he put in a new master cylinder and see if the
problem goes away.

TOM: If it does go away, then you know the master cylinder was, indeed,
starting to leak. If the brakes do exactly the same thing with a new master
cylinder, then we'd have to conclude that their diagnosis was correct and
there's nothing wrong.

RAY: Then, since they're the dealership, and they've already proven they're
nice guys (by not charging you before) they can put the new master cylinder
back on the shelf and only charge you for the labor. That's what I would ask
them to do, Ellen.

* * *

TOM: Is warming up your car actually BAD for it? Does slamming the door really
make a difference to the life of your car? Should you "save the brakes" by
shifting into a lower gear to help you slow down?

RAY: You'll learn the surprising answers to all of these questions, and more,
in our new pamphlet called "Ten Ways You May Be Ruining Your Car Without Even
Knowing It.'

TOM: It's our guide to making your car last forever.

RAY: Become an instant know-it-all. Order your copy of "Ten Ways You May Be
Ruining Your Car Without Even Knowing It!" Send $3 and a stamped (55 cents),
self-addressed, No.10 envelope to Ruin No.1, PO Box 6420, Riverton, NJ

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