What Pushes Brake Pads Away?

Dear Car Talk:

What moves the brake pads away from the brake rotor when you remove your foot from the brake pedal? -- Jaime

Not much, Jaime.

First of all, when brakes are working properly, the difference between the brake pads when they’re “away” from the disc rotor and when they’re touching the disc rotor is a few thousandths of an inch. That’s thinner than a Kate Moss (that’s the mechanical engineering term, I think). So the pads don’t have to move much either way.

When you step on the brake pedal, hydraulic fluid applies hundreds of pounds of pressure to the brake caliper pistons. Then, each piston moves the pads on that particular wheel toward the disc rotor. There’s a seal around each caliper piston that keeps the fluid from leaking out. And that seal has a sort of physical memory, and when you stop applying pressure, the seal naturally wants to go back to its original shape -- and take the caliper piston a few thousandths of an inch with it.

I suspect that the vibrations involved -- going over bumps, engine vibrations, any slight warp in the disc rotor -- also contribute to pushing the pads back from the rotors. And as I say, they don’t have to go far. In fact, if they were more than a few thousandths of an inch away from the disc rotors, when you stepped on the brake pedal, it would sink to the floor.

That’s why whenever we do brake work on the car, after we put everything back together, we have to return the calipers to their proper position, just a few whiskers away from the rotors.

And we do that by getting in the car and stepping on the brake pedal. The first time we step on it after a brake job, the pedal goes to the floor, but the fluid pushes the piston a little closer to the rotors. When we step again, the pedal goes maybe 80% of the way to the floor,and the pads get closer. Eventually, after five or six pumps, the pads make contact, and that’s when we know it’s OK to give the car back to the customer.

Before we figured that out, we lost a couple of garage doors and a soda machine as customers were driving out of the shop after brake jobs.

Todays Car-o-Scope

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