Technical Supplement No BKTSH001 - Caliper Overhaul

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Workshop Manual Reference

Section H-1-14 80, 95, 100 and 110 models only

Compiled: Barry Kensett

This revision 18 January 2014

 

Overhauling Front Brake Calipers - Disc Brake Cars

The workshop manual is a little vague with respect to details of overhauling front brake calipers and the following notes and diagrams may help.

If there is a possibility that the pistons are seized in the bores, consider using the hydraulic system to displace the pistons. Remove the pads and gently pump up the brakes until the pistons have been dislodged; they will not come right out with the disc in place. To prevent loss of fluid there are hose clamps available in motoring shops (e.g. Halfords) which can be used to nip the flexible hoses. Personally I don't like that very much and I keep a piece of wood which is cut to fit between the base of the driver's seat and the brake pedal; if the pedal is held down the port is closed off and minimal fluid then escapes. Remove the hydraulic connections and the caliper in accordance with the manual. Blank off the connections to keep dirt out, a bolt with the right thread is adequate. To fully remove the pistons do not try levering on the flange of the piston - on some of them the flange is quite thin and weak and combined with the possibility of tilting the piston and jamming it, there is a risk of damage to the flange. It is usually quite simple to put a foot pump onto the bleed nipple (open it of course) and a stab on the pump will pop the pistons out. Use a cramp if one piston is a bit stickier than the other to control them (Figure 1).

Be very careful when removing the boots to ensure that the brass rings that hold them to the caliper bodies (Figure 2) are not damaged as they are not supplied in the rebuild kits.. It is usually best to cut the boot away and then carefully lever the brass rings out. The brass rings on the pistons (fitted to some cars) are less important as the new boots now have a lip which will fit into the groove without the rings.


Even with care there is sometimes a slight bow in the brass rings and it is worth turning a washer to make a mandrel over which it is easy to dress back to shape. (Figure 2) The mandrel is also used to finally push the rings in place.

Inspect the pistons for damage and corrosion. I have come across three types of piston so far on P4's. Some are cast, have chromium plated outer surfaces and have a fairly thick seal retaining flange to the full diameter of the piston; these do not have brass retaining rings on the piston. The second type are turned and have brass retaining rings fitted to retain the boot, the flange is a smaller diameter than the piston to let the rings in. The third type is turned and has a shoulder rather than a groove on the end of the piston, the pad base forms the other side of the groove for the boot. It is essential that the outer rim of the piston is not corroded and it must have a smooth surface as this is the sealing face which slides against the sealing ring. The pistons are often corroded from inside and out. Traditional brake fluid is hygroscopic, i.e. it absorbs water and this water collects in the brake system and causes corrosion of steel parts. It is also possible for water to get into the cylinders from the outside once the boots have hardened or split with age. In Figure 4 the corroded piston on the left is scrap. The piston on the right (Figure 3) is not corroded and the sealing face will polish up well. New pistons are available or can be made by those with the knowledge and facilities. They can be turned from bar, preferably stainless steel and the outer face must be ground and polished to the precise size.

The bores of the calipers are generally in better condition than the pistons being cast iron and less prone to attack. The surface finish must be good as the piston slides in the bore but it need not be polished as it is not a sealing face. Usually a light polish with some well worn fine emery cloth will clean the bores; if they will not clean up then it will be necessary to find a better pair. Never try to split the caliper. Cleanliness is essential when handling the components and mineral oil must not be used. The bores should be flushed out with methylated spirit and the drillings flushed through and blown out - preferably with an airline if one is available.

To rebuild the calipers a kit of parts as shown in Figure 5 is required; this comprises piston seals and boots, new pads and retaining pins and clips. The assemblies should be built up using red rubber grease specially formulated for brake system use. This seem to be a little difficult to find these days probably because garages don't have the skills to rebuild cylinders these days, they just fit new ones which is a bit more difficult for us.


Fitting the boots and pistons is a bit tricky; I remember seeing them built up at Girling in the late fifties and they put the bits in before they bolted the caliper halves together. We have to work through the slot with a load of bits which would prefer to be on the floor rather than in the cylinders. The boots should be fitted to the calipers first, put the brass ring inside and then tuck the boot outer rim into the groove in the caliper body. Now the tricky bit ! The brass rings must be pushed down inside the greased boot which is then trapped by the lip entering the groove and stopped from coming out by the ring. The ring must not be bent while doing this as it will just pop out again. I find it best to hold the ring down with a pair of internal calipers (Figure 6) and then work round the ring pushing it down with a smooth bladed small screwdriver while following up with the calipers. Even when it looks as though it is down, hold it for a minute or two until the rubber has relaxed into its correct position otherwise the ring just pops out. Once it's in properly it stays in. (Figure 7). The first one will take at least an hour and subsequent ones ten minutes each when you've got the hang of it so stick with it and don't be tempted to leave the rings out. Finally slip the ring mandrel inside the boot and clamp it up to ensure that the ring is fully bedded. (Figure 8).


Fit the piston sealing ring into its groove, lubricate the piston surfaces and rest it on the boot. Using a smooth ended hooky bit of brass wire carefully stretch the boot over the piston (Figure 9) and slide the piston down ensuring that it goes in square and does not jam. Once it enters the seal it should slide down smoothly. Finally feed the boot into the groove around the piston with plenty of rubber grease to help keep the water out and stand back proudly (Figure 10)


Check that the disc face is not corroded or glazed, refit the caliper, the pads and retaining pins as instructed in the manual. Remove the pedal strut and fill and bleed the system. Pressurise for ten minutes and check carefully for leaks.



©B.Kensett 2023