Absolute cleanliness is essential when dismantling and reassembling these units, so clean out the outside well before beginning. When dismantling ATE type brake calipers, mark the pistons in order to be able to refit them in the same position on the rim step. Remove any dust cover retaining rings and the dust covers themselves, then put a piece of wood about 1/2 inch thick between the pistons and push the pistons out of the caliper using compressed air by means of a hand or foot operated tyre pump, and apply the air at a low pressure. If you have a jammed piston, a local garage might help with a high pressure air line. Carefully remove the inner sealing rings with a small screwdriver making no scratches on the bore or damage on the groove and consider rounding off the corners of the screwdriver. Unscrew the bleed nipples and hose connections but do not try to separate the two halves, as to reassemble the assembly requires special equipment. Clean the various parts and connecting paths thoroughly with methylated spirit which will not affect the rubber seals, and with a lint-free rag (use a hair dryer for the fluid pathways to make sure all the methylated spirit is removed). Inspect the parts carefully; if any cylinders are scored or scratched the entire cylinder housing must be replaced along with any other damaged or worn parts. Reassembly is simple, coat the different parts with brake fluid before reassembling, the inner sealing rings should be installed into their respective grooves in the correct direction, and then the pistons with the large ends should be installed inside the housings. For ATE type brakes, make sure that the outer end of the piston is inclined by an angle of 20 degrees (use a special tool for this purpose if necessary). If replacing a piston, compare the new one with the old and make the same marks.