Help with rear brakes, what am I doing wrong? Options

[Kevin@ojai.net]

So, a few minutes ago I tried replacing my rear brake pads with out success. I was unable to retract the pistons far enough into the calipers to fit the new pads in. All I did during my failed attempt was remove the old pads, then pry against the pistons. This process has worked for me on other vehicles, but I had almost no luck on the 914. I was able to retract the pistons a little, but not nearly enough to get the new pads installed. What am I forgetting to do? What am I doing wrong?


-Kevin

[davep]

Just to get you started, in addition to all the good advice so far. I would start by removing the caliper from the car and using a big bench vise to hold the caliper while working on it. I like to use old brake line ends with the line cut off flush and welded closed as plugs on open lines and ports. I'd open the bleed valves and drain the caliper, then blow out any more fluid with compressed air. I use small drill bits to clean the inside of the bleeders until you can see daylight coming from the small hole in the end. Scrub the caliper as clean as you can, and rinse with rubbing alcohol or use brake cleaner. Remove the piston dust boots carefully. Clean and flush the exposed piston carefully. I use small brass brushes

The outer adjuster is supposed to have a brown plastic cap with a 13mm hex head. Carefully remove the cap if it is there. Underneath is the threaded adjuster with a 13mm jamb nut. Loosen the jamb nut, but don't remove; there is a C clip that is supposed to keep the jamb nut from coming off. You should now be able to turn the adjuster with an Allen key; clockwise threads the shaft into the piston and pulls it into the bore. In the event that the piston is seized, very common, the adjuster just seems to spin and does nothing. If this happens, then very seriously consider talking to Eric Shea about his exchange program. What happens is that the internal mechanism has become unlocked from the piston. The mechanism has a lock ring that fits into an internal groove. If you turn the adjuster counter-clockwise enough you should hear an audible snap as the mechanism is repositioned in the groove. You can try again to see if the piston will retract. If you can get the piston to move, push it out about a half inch and clean what you can. Then retract until the step is flush with the bore. Reinstall the dust boot, then retract fully until the piston face is flush with the bore.

If the piston does not want to move, you can try this procedure to push the piston out. Carefully remove the C clip; I use two small screwdrivers to put on both sides until it is loose, then put one screwdriver into the gap and pry off; if you are not very careful you can loose this clip, so go slow and easy. Then unscrew the jamb nut to the end of the adjuster. Tap on the adjuster & nut with a hammer to drive the piston out. If it moves, very good, and when the nut bottoms out just unscrew the adjuster and try again. Eventually you can drive the piston completely out. I often use a green kitchen scrub pad to remove rust from the pistons. If the piston is pitted, you need to replace it; call Eric for a rebuilt caliper. If you get this far, you will probably have to get the rebuild kits and replace all the seals. In particular, the small O-ring on the adjuster shaft tends to disintegrate and will leak if not replaced. While many people are able to do this, it is not a trivial task and should be left to experts. That is why the factory said never to disassemble the calipers; they didn't trust the dealership mechanics to do the job properly let alone a customer.

The inner adjuster is also under a cap; it is supposed to be a round flat cap with an Allen key hole. Use a drift that about covers the cap, and hammer firmly to try and break the rust locking it on. Hopefully an Allen key will remove the cap after the hammering has loosen it. If not, and this is very common, you will need a cold chisel and the hammer to remove it. A dull chisel is best so that it doesn't cut the material off. Suitable application of heat and penetrating oil etc is also reccommended. Note that there is supposed to be a copper washer under the head of the cap. There is supposed to be a gear with an Allen key hole within the adjuster bore. If not, you will have to check the other caliper. I would hazzard a guess that most cars will now only have one adjuster for the two calipers; this was true almost 20 years ago. Because this adjuster is geared it works in reverse to the outer adjuster. You also have the parking brake lever to assist in moving the inner piston. The lever has an internal cam that acts on the end of the internal adjuster shaft moving the shaft and the piston together. Since you don't have direct access to the shaft to tap with a hammer, you are limited to turning the geared adjuster clockwise, then pulling on the lever to force the piston out.

The internal mechanisms are designed to retract the pistons by spring action after the brakes have been hydraulically activated and released. In the front calipers the retraction mechanism rides on a shaft which it grips securely enough for applying a retractive force, but slides easily enough to be forced out as the pads wear, and forced in to be reset when new pads are installed. On the rear, these mechanisms are threaded to the adjuster shafts. They only have a small amount of travel, say a few millimeters. If you do not constantly adjust the venting clearance (0.004")as the pads wear, say every 10,000 miles, then the mechanisms will be pulled out of the pistons. It could happen that the parking brake is not effective due to the mechanism becomming unlocked from the piston. You need to turn the adjuster clockwise until the mechanism snaps back into place, then the lever should move the piston.