Difficulty Level 5
Adding air to your tires is level one
Rebuilding a 911 Motor is level ten
Rebuilding Brake Calipers
Premature or uneven brake pad wear is often a sign of sticking caliper pistons. If you experience this, or if the rubber seals around the pistons are torn or if there is any seepage of brake fluid around the pistons, its time for a rebuild.
The procedure is quite simple and needs no special tools other than a compressed air source. This article shows you how to replace the caliper seals in the Porsche four-piston calipers, but applies generally to the two- and single-piston calipers as well. All photos are from a 944 Turbo. A tech article describing the procedure for 911 two-piston calipers is available on the Pelican Parts site.
Make sure you have the correct rebuild parts for your particular calipers. At a minimum, you will need one dust seal and one pressure seal for each piston. Note: Porsche refers to these parts by various other names, such as dust boots, scraper rings, etc. It is generally not necessary to replace the pistons unless they show signs of wear.
Jack the car up and place jack stands appropriately to securely support the car and remove the wheel (fig. 1).
Remove the brake pad retainer. On these calipers, the pads are held in by a “clover leaf” retainer. Squeeze the center of the retainer with pliers until the top loop releases (Fig. 2). Then swing it down (Fig. 3). Other calipers use pins that go across the calipers and through holes in the pads. Remove the clips that hold the pins and slide out the pins.
Slide the pads out (Fig. 4). If you can’t pull them out, use a large screwdriver to pry the pad away from the rotor a little bit (Fig. 5), then it will come out easily.
The next step is remove the brake line from the caliper and unmount the caliper. Depending on your brake line fittings, sometimes its easier to remove the brake line from the caliper before unmounting the caliper, sometimes after. If the fitting turns independent of the line, remove the fitting first. If the fitting is crimped to the line, just loosen it slightly for now. Remove the two caliper mounting bolts (Fig. 6) then lift the caliper off. Rotate the caliper to unthread the line if its the crimped type (Fig. 7). Careful: brake fluid will seep from the line and from the caliper, so have some newspaper or rags in place. If you have an appropriate fitting, you can cap off the brake line. Another technique is to wedge a stick between the front of the driver’s seat and the brake pedal. Adjust the seat position so that the stick depresses the brake pedal through approximately 50% of it’s travel. This closes off the valves in the master brake cylinder so brake fluid won’t flow into brake lines.
If you haven’t done so already, clean up the caliper as much as possible. Inspect it to determine if you need to replace any other parts such as bleeder bolts, bleeder caps or crossover pipes.
Remove the dust boots from around the pistons by simply prying them out. Use a plastic tool if necessary to avoid scratching the walls of the piston. Note: Some calipers use a second piston seal as a dust seal. This seal is down in the wall of the cylinder bore and can’t be removed until after the piston is removed (Fig. 8 – different caliper).
The next step is to remove the pistons from the caliper bores. The easiest method involves removing and replacing one piston and seal at a time. A bit more difficult is to remove all the pistons at once, particularly for the 4-piston calipers. In either case, you use compressed air to force the pistons out, and C-clamps to hold in the ones you don’t want to come out.
Place the caliper in a pan on the workbench, accessible to a compressed air source and pressure regulator. Screw a hose fitting into the caliper and connect the air hose to the fitting (Fig. 9). The air hose should have a regulator close to the caliper (gauge is not necessary).
The trick is to control the movement of the pistons so they SLOWLY reach full extension. If you’re doing one at time, use C-clamps and an old brake pad and/or wooden blocks to hold the other pistons in place. Then SLOWLY turn up the air pressure and watch the unclamped piston. As soon as you see movement, back off the air pressure. Carefully regulate the pressure to get the piston to extend further (Fig. 10). Your aim is get the piston right at the verge of popping out, at which point you should be able to pull it out. Once one piston is out, you won’t be able to pressurize any of the others. If you want to get all the pistons out at once, gently insert the freed piston back into the bore and lightly clamp it, just tight enough to seal it. Then apply more air till the next one pops. Repeat till all the pistons are free. NOTE: Its important to carefully control the air pressure. Too much and the piston will shoot out of the bore, possibly damaging its surface.
As pictured, I prefer to remove all the pistons so I can thoroughly clean and inspect the caliper body, but you can rebuild one cylinder at a time. Once the pistons are out, remove the rubber pressure seal rings from the bores (Fig. 11). Use a plastic tool if necessary, to avoid scratching the walls.
Thoroughly clean the pistons and cylinder bores (Fig. 12). Inspect the walls of the pistons. If scratched, worn or have any signs of rust, they will need to be replaced.
Coat the new pressure seals with brake fluid and press them into the recess in the bore. If you have the recessed type of dust seals, coat them and install them now. Mine have the external dust seals/boots that fit onto a groove in the pistons (Fig. 13), so its convenient to fit the dust seals onto the pistons prior to pressing the pistons into the bores.
Coat the walls of the bores and pistons with brake fluid. Press the pistons into the bores. Make sure you press the pistons in straight! Its a tight fit, but I was able to push them in by hand. If you can’t, use a block and C-clamp. If the face of your pistons are notched, click here.
Seat the dust seals into the counterbore around the pistons (Fig. 14 ) if you have the external type.
You’re done with the rebuild process. Now re-install the calipers on the car, connect the brake lines and install the pads. Thoroughly bleed the system. Enjoy them!
Installation Notes For Brake Caliper Piston With Notched Face
Some calipers utilize pistons with a notched or recessed surface that presses on the brake pad. The purpose of the notch is to attempt to even out the pressure applied to the pad, since the leading edge of the pad would otherwise do more work than the trailing edge. When installing these type of pistons, orient the notched portion such that it faces the leading edge of the pad. This would generally be facing upward if your caliper mounts on the leading side of the rotor, or facing downward if your caliper mounts on the trailing side of the rotor. To be more technically correct, there is a Porsche alignment tool to set the orientation, but the above guideline will get you pretty close.