I began this series on Jaguar Mark II brake rebuilding with last month's article (Jaguar Mark II Oil Change & Brakessee www.southernwheels.com and click "Archives.") Then we removed the calipers from our '67 Mark II and now are ready to rebuild them. They were completely locked up from years of sitting. The Jag's brake system was pretty advanced for the time period including 4-wheel disk brakes with a vacuum power booster.
The system consists of one two-piston caliper on each wheel that straddles the disk and houses a pair of friction pad assemblies, each comprised of a pad and securing "keeper" plate. The friction pads have a metal plate riveted to the back side of them, with a slot in the plate to guide and lock the pads to the pistons. A "keeper plate" on the caliper fits over the pads to hold them in, and is attached to the caliper by one bolt/nut. The caliper has two cylinder blocks, one on each side, holding the piston assemblies. These operate the friction pads. The pistons have two grooves to accommodate the rubber seals. The innera pressure seal, and outera dust seal. (Front calipers have a larger bore.) There is a refractor pin in the center of the caliper bore and the pistons have a corresponding hole in them. When the pistons are pushed into the caliper bores, the pin aligns them. The pins are also designed to push the pads toward the rotors as the pads wear down.
CALIPER REBUILD: Before the calipers were rebuilt, we shot some detailed pictures, made notes and bagged and tagged all of the bolts. We also referred to our shop manuals for rebuild instructions and parts book to see if we had any missing parts. Before disassembly, the calipers were cleaned using brake cleaner and a wire wheel, then using air, the grit was blown away from all crevices. The emergency brake assembly is attached to the rear calipers. It is mechanical, not hydraulic, and has two pads that move via a hand brake. Our mechanism worked and the pads were good, so we just cleaned it up and removed it from the caliper.
PAD REMOVAL: To remove our caliper pads, we removed the keeper plate. The pads have a hole at the top of the metal backing plate and by inserting a wire through the hole it is easy to pull the pads out.
INSTALLING FRICTION PADS: Insert the new pads into the caliper so that the slot in the metal plate attached to each pad engages the stud button in the center of each piston (pad end). With the pads in, put the keeper plate over the top of the pads and bolt to the caliper.
PISTON REMOVAL: To remove the pistons, we removed the four bolts from the cylinder blocks, the rubber dust seal from the groove around the cylinder block face and removed the cylinder blocks. This left the piston exposed. Ours were frozen, so it took time soaking them with brake cleaner before they could be pulled. Now with the calipers disassembled, we could clean everything and lay the parts out on clean white rags. Our cylinders were not scored and did not have to be re-sleeved.
TO ASSEMBLE PISTON SEALS we used Jaguar-recommended Castrol GT-LMA (low moisture activity) brake fluid. Using grease to assemble can swell the rubber seals, a common problem in vintage British cars using British rubber.
With the seals lubricated, we put them on the pistons, pressure seal inner and dust seal outer. Then we pushed each piston into its cylinder, aligning it with its refractor pin. Care must be used not to twist the seals. With the pistons in, we pulled the outer rim of the rubber dust seal into the groove around the cylinder block face of each piston, then reassembled the cylinder blocks to the calipers. We reattached the emergency brake assembly on the rear calipers. To finish, we masked off the pads and rubber seals then sprayed on a couple of coats of hi-temp cast. The calipers are ready to put back on and in an upcoming issue we will complete the brake overhaul.
Winter's over! Get the cars out, and keep 'em driving!