SWFL Posted February 18, 2023 Report Share Posted February 18, 2023 I'm going to be doing a brake and wheel bearing repack job. I'm a bit concerned as the calipers look like a big wheel cylinder to me, with pistons on each end. I don't want to make my truck undrivable until I have some spare parts on hand. My pads have plenty of life, they and the rotors just need deglazing. I do have new pads, but I'm worried about pushing in the pistons, as I have no idea what the bores look like inside. I have some rebuild kits, but I'm not sure if the seals are in the caliper bore, like most calipers or on the pistons like most wheel cylinders. My 620 is a 1978 with disc front brakes. The reason I mentioned wheel cylinder, is that's what the calipers look like to me, with a goofy frame to mount to the knuckle and hold the pads. Quote Link to comment
Stoffregen Motorsports Posted February 18, 2023 Report Share Posted February 18, 2023 If they are bad inside, then compressing them isn't going to harm them more than they are already. Compress them with the bleeder open. This way, any junk inside the caliper will squirt out and not contaminate the fluid in the hoses. 2 1 Quote Link to comment
datzenmike Posted February 18, 2023 Report Share Posted February 18, 2023 The caliper has two pistons on the one side of the caliper, one pushes outward on the surrounding yoke effectively pulling in the fat pad. The other piston pushes inward directly pushing on the other pad. The cylinder has little wear as only a rubber seal actually makes contact with it. Anything behind the piston is air tight and filled with brake fluid. I would not worry about cylinder wear. There is no need to retract the pistons, other than to wiggle them enough to remove the pads if you have lots of pad wear left just pop them out and rub lightly with a circular motion on some sand paper on a glass pane to remove any high spots. The rotor, do the same with a small piece of wood wrapped with sand paper. Glazed is an over used term. The rotor should look like a mirror, it's fine. If lightly rusted the pads will polish the rotor by driving. If it has noticeable high or 'hot' spots or the pedal pulses during braking, have them turned. If replacing the pads, unless perfectly smooth you may as well have the rotors turned down. If retracting the pistons always open the bleeder. The brake fluid in the calipers is the most abused by heat in the system. You don't want it being pushed back into the master cylinder and being reused. Before driving on the new pads and possible the new turned down rotors be sure to pump the brakes several times to bed the pad against the rotor. Failure to do this means the first time you use the brakes it might take more than two pumps of the pedal. I've done this and it's scary when the brakes don't apply. If one pad is severely more worn that the other it's possible the caliper is stuck and not free to self center itself. With pads still in have someone press on the brake pedal. If the caliper appears to be forcing the rotor over to one side this is probably the case. While the pads are out you can tap the caliper back and forth to loosen. You may have to remove the caliper to wire brush the rust off. Use lithium or silicon brake grease to lubricant the sliding contact surfaces. Quote Link to comment
SWFL Posted February 19, 2023 Author Report Share Posted February 19, 2023 #datzenmike Are the piston seals mounted in grooves in the caliper bore, like a normal closed end caliper, or are they mounted onto the pistons, like a wheel cylinder? I assume the pistons are phenolic or steel with a phenolic lining or end? Quote Link to comment
datzenmike Posted February 20, 2023 Report Share Posted February 20, 2023 Square edge rubber seal goes in outer end of the cylinder. The piston also has a groove around the outer edge to receive the inner diameter of the dust shield. The outer edge of the dust shield is attached to the cylinder. The piston can extend out of the cylinder as the pad wears down but the dust shield stays in place. 1 Quote Link to comment
Stoffregen Motorsports Posted February 20, 2023 Report Share Posted February 20, 2023 The pistons are almost always stainless, though I have seen chrome plated pistons. Stainless will almost never pit, so even if there are pits in the bores, the calipers are still rebuildable. Sometimes pitting occurs between the two halves of the caliper, around the sealing o-rings. A light sanding can make this area new again. Quote Link to comment
datzenmike Posted February 20, 2023 Report Share Posted February 20, 2023 The early '80s Toyotas 4x4 were chrome plated steel. Had to combine several sets to get two calipers I could rebuild. Just a rusty mess inside. 1 Quote Link to comment
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