Brake Piston Repair

Last week-end before the Palo Alto Concours I asked John Thornton to drive my car and recommend some changes that would improve the handling. He recommended new shocks, brake pads and replacing the seals on the brake caliper pistons.

Pads are replaced by pulling of the retaining pins and sliding the pads out. Worn pads take less space between the piston than the disk than new ones. A large screw driver can be used to pry the pistons back into the cylinders providing space for thicker new pads.

My first caliper was used for learning the process. By the time I got to the last it was straight forward if tedious.

In general,

  • One wheel is done at a time. After that wheel is done and remounted and the pads replace, then do the next wheel. This insures you can use the brake system to help free a stuck piston for any wheel without needing to bleed the entire system.
  • The pistons are removed one side of the caliper at a time. After the piston on that side is complete, then attention is turned to the other piston.
  • Conventional wisdom says, do not split the calipers. It is difficult to put them back together without them leaking. An exception may be the rears which have an outside line and two bleed screws.
  • The pistons need to be oriented before they are put back into the cylinders.
  • A pressure bleeder is a big help in bleeding the brakes with just one person.
  • When doing the final adjustment, the pads should be removed, the pistons extended with the brake pedal and then the pistons pushed back just enough to put in the pads. If this is not done, the brakes will be soft as though there is still air trapped in them.
030620-2726 A piston kit has parts for one caliper. Including a square seal that fits in the cylinder, a rubber dust boot and a steel retainer ring to hold the dust boot on the piston and caliper. In addition there are two retaining clips for the pins that hold the brake pads in place.
030626-2918 I jacked up the car and put four jack stands under it. Now all four brakes can be done.
030628-2926 The procedure for putting new seals in the pistons normally is to remove brake pads, push the pistons to the brake disk by pumping them out with the brakes and then removing the the caliper. Then restrict one piston from movement. Blow out the other pistons with air pressure. Clean this one up and replace with new seals.
030628-2930 A 'C' clamp is being used to hold one piston in the caliper so the other piston can be blown out with air pressure.
030624-2931 Here is the setup ready to blow the next piston out of the caliper. It is well wrapped with rags to prevent brake fluid from being blown all over the shop and you when the piston comes free.
030628-2927 The piston has come out and both the cylinder and piston can be cleaned up.
030628-2932 Here is what all of my pistons looked like when they popped out of the caliper. Some 400 grit emery paper polishes up the surface of the piston so it looks new.
030626-2922 Here is the out side of the piston. This side presses against the back of the brake pad. Notice the section of the outer raised portion which has a flat cut in it. The flat must be oriented with respect to the travel of the disk.
030626-2923 Here is the back side of the piston. The center hole rides on a pin in the center of the caliper opening.
030628-2929 Here is the piston after clean up, reinstalled in the cylinder with dust boot installed. See the text with the picture for some discussion of orienting the piston.

Bleeding the Brakes

This discussion covers more than the 'bleeding' the brakes which normally just considers removing all the excess trapped air from the master cylinder, lines and calipers.

A pressure bleeder works well for this application. It is a pressurized tank that is screwed on to the brake reservoir. You may need to close the over flow tube with some small vise grips if there is one. The early cars do not have this hose. Pour fresh brake fluid into the tank of the bleeder. Screw on the cap from the bleeder to the reservoir and pressurize the tank with the pump handle to 10 psi.
030626-2916 Ate super gold brake fluid. I changed to this brake fluid because I wanted to purge the old Ate Blue. The color change shows the system is flushed.
030629-2937 See the text for the procedure to eliminate spongy brakes. The final product, new pads and firm pedal.
Last modified: Sat, 28 Jun 2003


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