What force causes the brake pads to back off the disc? It can't be the piston backing into the caliper, due to the large spring inside of the caliper pressing it out.

After you release pressure on the brake caliper, the spinning of the rotor causes it to back off just a little bit. They may still drag, but they aren't still going to be clamped on.

That spring you must be talking about is probably the self adjuster for the rears. I still think that the release of the brake pedal causes a small amount of fluid to be sucked back into the master cylinder bore, and this sucking back might cause the caliper piston to move back just a millimeter or less, which is enough to release pressure on the rotor.

Joe is right. I had a caliper freeze up on me because the previous owner neglected grease the caliper slides. Froze up after applying the brake gently, it heated up enough I could smell it within 100 yards. I got home by stabbing the brake quickly a couple times once I got rolling. Stabbing the brake it backed off the piston enough so it wouldn't heat up. I drove about 45mph tops and check to see if the one was dragging I put the car in neutral. All was fine and I got home.

It's very important to grease the slides with high temp grease, It's a must now for all my cars on every brake pad change or I happen to take the caliper off for whatever reason.

[This message has been edited by Tigger (edited 04-08-2002).]

the pads are in continuous contact, even when there not applied. Normal operation.

I don't know if it happens on Fieros, but, on some cars the rubber seal on the pistons helps the pads move back a little bit.

I'll go with Roger's reply. So if the pads are in continious contact, but at varying pressure levels, how do the pad to piston clips help?

Try this:
When the pedal is depressed the fluid pressure is increased form 0 to a he!! of a lot of pressure.
When the pedal is relased, all that honking pressure goes back to 0 and the piston returns to the start position. (away and off the rotor)
Cleanliness and smooth and clean contact surfaces, the ones where the pads rest at the ends make metal to metal contact as well as the mounting pins with a good lube.

When the brake fluid pressure goes back to zero, I can agree with a decrease in pressure on the piston and pads, but no one can show the pads lose contact with the rotor unless the rotor is badly warped.

I believe that the piston to caliper body o-ring does most of the "pulling back." This is a square cross section o-ring, so it prefers to be resting on its flat surfaces. When you press the brake pedal the o-ring rolls slightly onto its square edge, compressing itself. When the pedal is released, the o-ring wants to decompress to rest on its flat sides, so it rolls back into the caliper slightly, pulling the pads back.

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Joe Furedy
'88 Formula

theres never any clearance between the pads and rotor. When your not applying brakes it has zero pressure or very nearly so. Thats one reason disk brakes are so much better than drum. usually not even enough clearance for water to get between them. Your constantly wearing the pads against the rotor too, but that is so minor, not even worth mentioning for normal wear n tear. If you never used your brakes at all except when your already stopped to keep the piston out, you would still eventually wear out the pads. Jack up your car and try spinning the wheel and it wont go far, you can feel the resistance of the pad/rotor. Only way to get them retracted enough to not rub is taking the wheel and pads off and physicaly pushing the piston back in caliper.

I'll take another shot at it...

When the brakes are applied, we know the caliper piston is forced out from brake line pressure and the inner brake pad starts to contact the rotor, the clamping action of the caliper takes over and moves it along the slides until the outer pad makes contact.

When the brakes are released, I'm pretty sure the piston retracts a slight amount due to the release of pressure on the brake fluid creates a sort of backpressure and the clip that holds the inner pad brings the pad off of the rotor surface and keeps it from wearing the top edge because it would always be lying against the rotor at an angle creating uneven wear on one edge of the pad.

I'm almost certain the piston retracts a certain amount because when I gently let off the brake it stayed clamped (my post above). When I stabbed the brakes at least the piston side pulled back enough for me to get home. My problem may even had been a bad caliper piston along with bunged up slides but I replaced the calipers because they need to be rebuild/replaced.

The outer pad technically stays in contact with the rotor because nothing mechanically can pull it away. Usually with rotors being out of round to a point, they push the pad away within a few rotations, if not vibrations and whatnot will eventually persuade it further back.

So yes I'm sure the outer pad "technically" stays in contact but no pressure so the wear is minimal compared to actual braking.

[This message has been edited by Tigger (edited 04-09-2002).]

"the release of pressure on the brake fluid creates a sort of backpressure"

Yeah, like 0 psi. No way is the release of the brake pedal going to create a vacuum in the brake line.

if I remember my high school auto shop,the seal in the caliper helps pull the pad off of the rotor. When I used to drag race we felt it was better to use drum brakes, then we could be sure the pads were free.
John

mountianman is correct. the seal in the caliper is square cut. it deflects out about .002" to .003" when brakes are applied. when pressure is released the seal returns to original position pulling piston in slightly .002" to .003".

Cool, I'll go with the seal pull back. That takes care of the piston or inboard side pad.
What about the outboard side pad? Why is it thinner new than the inboard side pad and why is the wear indicator (SCREEEECH!!) on the outboard pad?

I wasn't speaking gospel, just my thoughts on maybe why stabbing the brakes got me home with a frozen caliper. I was thinking that even though it sounds obsurd, why because if you compress a piston in a closed cylinder of oil it will spring back ever so slightly because the fluid is slightly compressible.

On disc brakes, the calipers' piston seals are designed to retract the piston slightly, thus allowing the pads to clear the rotor and thereby reduce rolling friction.

Go with the seal pull back because that's the answer you are looking for...

For your next answer it's because the outer pad wears the fastest and the brake pad company wants to screw you into buying new brake pads sooner. No, maybe it's so the wear bar on the outer pad squeels before the inboard because the inboard pad is in contact with the rotor a millisecond longer than the outer pad each time the brake is applied.

Nothing, there's just no longer any force pushing it IN, and that's enough...

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Ben Cannon
88 Formula, T-top, Metalic Red. (2:13.138 at Sears Point) "Every Man Dies, not every man really Lives"
88 Formula, Northstar, Silver, In-Progreess. -Mel Gibson, "Braveheart"