Stuck Brake Piston
 

I replaced the rear brake pads on my '88 260e today and found that one brake piston was stuck. I removed the rubber boot and lubricated the piston with brake fluid and was able to force it in with a C-clamp. After working it in and out, it seems to be almost normal again.

Should I start looking for a replacement caliper or will my fix be okay until the next pad replacement?

You'll know right away when you take it for a drive. If it pulls to the side away from the stuck piston, the piston is still binding and you will probably have to rebuild the caliper or get a rebuilt caliper. If it tracks straight when you brake hard, you are golden:D

Get caliper rebuild kits and rebuild (that is, replace seal and dust boot) on both calipers on that axle.

The only reason they stick is bad boots or seals. There are several causes (old age, for intance, when the rubber gets hard), but if it stuck, it will stick again until properly cleaned and a new piston seal is installed.

Also, on later models with floating calipers, check for play in the guide rods and bushings, too. Usually shows up as tapered brake pads, thinner at the front than the rear, too. If there is any perceptible side play, replace them as loose floating calipers cause major reductions in braking effeciency.

Make sure you do not run the rotors too thin -- if there is a distinct lip at the edge of the rotor where the pads don't quite cover it, replace the rotor, it's too thin. Usually, by the time you've worn out the second set of pads on a rotor, the rotor is gone.

When the rotor gets too thin, the piston extends too far and will cock in the caliper bore. This causes it to drag the pad on the rotor and get too hot, and the heat cooks the rubber dust boot and piston seal. Dirt gets in from the torn boot, and the piston seal becomes hard, causing the pads to stay out instead of retract.

It's an easy fix.

Peter

The stuck pistons that i have worked on all had good seals. I think they stick from corrosion cuased from old brake fluid. Brake fluid absorbs moisture and it should be flushed about every two years, preferably on a dry day.

Yes, John, but to clean the piston and caliper you have to take it apart, and at that point I think it's foolish not to replace the rubber parts!

Peter

Peter,

What do you use to get the heat shield back onto the piston, and seated .004 below the face of the piston, without mangling the heat shield? I understand that there is a high dollar tool especially for this purpose?

A piece of thin wood. Stick it through and lean on it. Same one I use to push the pistons back in.

Make sure you have the sheild aligned correctly first, though, as it is impossible to get it back out intact, at least for me.

Peter

That might allow you to get the heat shield flush with the piston, however, the manual calls for the piston to stick out past the heat shield by .004 or more. Do you not worry about it?

Trying to get it down below the face of the piston is almost impossible.

.004 is the same as zero when I'm working on my cars. :)

That piston needs to come out and both it and the bore need inspection. Corrosion on either is no good. I'm a bit of a risk taker and have been known to use steel wool to buff back down the corroded parts. Blasted them real good with brake parts cleaner and then wiped it all up with clean fluid. Never has caused me any grief since. I actually found brake jelly (used to be fluid) behind my pistons. There is NO WAY a flush could have cleaned that stuff out. It sure as hell could have prevented it though.

Rebuild Parts Source
 

Does anyone know where I can get a rebuild kit for ATE calipers? Fastlane lists a kit for the rears but the photo shows only the dust boots and not the piston seals.

I would call Phil ( FastLane ) direct.
He has ALL the answers. ;)

The kit only has three parts for each side -- head shield, dust boot, and piston seal. The seal is usually sitting inside the lip of the dust boot in the package, so may not show.

Peter