PeachParts Mercedes-Benz Forum - View Single Post

psfred · #7 02-03-2002, 03:35 PM

There is a better way than compressed air -- use the brake pedal!

Remove caliper, clamp the "unstuck" side, then step on the brakes hard. This generates more pressure that is safe to use with compressed air, and won't blow out as the pedal will hit the floor when the piston is free. If you get carried away with the compressed air and the piston is really stuck, you get quite a blast when it does let go.

The usual reason an MB piston is stuck is that someone left under-thickness rotors on the car, the backing plate on the pad jammed up on the anti-rattle spring, and the piston is cocked in the bore. They can often be freed just by tapping on the upper edge to straighten them out in the bore.

Don't bother to hone the bores, and never, never use abrasives on the piston. If they are rusty or corroded or have crud on them that won't come off with brake cleaner, get rid of them. The bore and piston are both chrome plated for corrosion resistance, and the coating is thin enough that honing or cleaning with abrasive paper will remove it. The caliper will again seize in short order. The piston will no longer seal properly on the bore, and won't retract properly if it isn't very, very smooth (shiny bright). I'd also not take the caliper halfs apart -- they need to be very closely aligned for proper operation, and getting them re-sealed can be a pain.

Caliper re-build is very easy -- pull piston, remove square seal with a plastic or wooden pointed tool, no metal picks!, clean well with brake cleaner (no abrasives, please), wipe down with brake fluid, coat new seal with brake fluid and insert in slot, align piston (the heat shield won't fit if the piston is rotated, easy to see), press piston back into bore, attach new dust boot, and clip heat shield into place.

The piston can be removed by prying straight up with two screwdrivers from the sides of the caliper using the dust boot lip.

Peter