From Minnesota MG Group
By Dave Braun
In the past we’ve talked about brake fluid types, the Pressure Differential Warning Assembly (PDWA) on dual circuit brakes, and the importance of maintaining your brakes and fluid. But we’ve never really discussed the brake calipers, and in fact, doing so is probably long overdue.
Brake calipers are decidedly different from rear wheel cylinders. Although both are hydraulic in nature, wheel cylinders simply expand, pushing a set of shoes against the brake drums. The brake drums are rotating with the wheel and the force is applied in a radial fashion. In single wheel cylinder systems the action of the rotation tends to tighten one shoe against the drum, while the other shoe is more or less trying its best to stay in contact with the drum. We call these the leading and trailing shoes respectively. The front calipers on the other hand are designed to push perpendicularly against a rotating brake disk (or rotor) which is installed in the same plane as the rotation of the wheel. The size of the pistons in the calipers is many times the diameter of the master brake cylinder piston. All things being equal, the volume of fluid moved by the master cylinder must be equalized between the master cylinder and the four (two in each caliper), much larger caliper pistons. As a result, the brake caliper pistons only move a small fraction of the master cylinder movement. This smaller distance and the larger surface area create a large force multiplication. Calipers use this powerful hydraulic force multiplication to clamp the brake pads against the disk (or rotor) slowing or even locking up the wheel depending on the coefficient of friction of the tire with the surface of the road. With the release of the brake pedal the rotation of the brake disk moves the brake pads an almost imperceptible distance from the disk resulting in a quick and sure application the next time around, and as a bonus makes the brake calipers completely self adjusting as long as the fluid is topped off in the reservoir occasionally. Prior to the advent of disk brakes, in order to cope with the additional braking force required in the front due to weight shift under braking, drum brakes with multiple wheel cylinders to make both shoes leading were used on the front of MGs.
The MGB caliper is of the non-sliding variety. This means that the caliper is fixed to the steering swivel and that there are pistons on each side of the disk to push the pads against the disk. In sliding calipers, there are only pistons on one side of the caliper and the opposite pad is perched against the caliper itself. On sliding calipers the entire caliper floats on slides in a sturdy bracket to create the movement needed for engaging the opposite side.
It goes without saying that the goals of the caliper is to contain the hydraulic fluid under static and pressurized positions; keep our constant enemies of dirt, moisture and acids from the internals; and provide an equal application and release of the force to the pistons which are in turn acting on the brake pads. To do all of this the bores in the brake calipers are carefully sized to the pistons, with allowances (and grooves) for the hydraulic seals which are rings with a square cross section. The pistons are highly polished to resist corrosion, and replacements are available in stainless steel. The dust seals are also rings, but have a pair of thin flanges at their edge which contact the piston and are held in place by a formed sheet metal press-fit retainer. The halves of the caliper can be separated, although it is typically not necessary, but you can get the internal seals if you decide to go this route. Be warned though that the workshop manual requires in addition to new seals, new clamping bolts and lock plates, neither of which are currently available.
As long as we were rebuilding Maggie’s brake system and switching to DOT 5 Silicone fluid, for which I assure a thorough cleaning and rebuilding of all the components, it was a good time to inspect the calipers, their pistons and seals, and replace and refurbish as needed. This meant Maggie was already on sturdy jack stands and well supported. The calipers are easiest to rebuild on the car, but only if you intend to leave the exterior of the calipers as found, which in Maggie’s case was pretty grungy. However to proceed in this manner simply siphon some brake fluid from the brake master cylinder reservoir to provide room in the reservoir for fluid as you push the pistons back into the calipers. Bend back the tabs on the bolts that hold the calipers to the swivel axle and remove the bolts and then clear the caliper from the brake disk and suspend the caliper from a wire or bungee so you are not stressing the brake hose. Remove the brake pads by bending back the long cotter pins, withdrawing them, and removing the anti-rattle brackets as well. The pads will now pull straight out of the calipers. When we inspected Maggie’s brake pads, there was still some life on them, but since we were completing an entire brake job, replacing them was a no brainer.
The old and new brake pads. Yes, there was still life in the old pads, and it was nice to see them wearing evenly. A sticking caliper piston often shows up with uneven wear on the pads. It is much easier to do maintenance rather than repairs as the ease of disassembly is usually better. You can see that the new pad is going to require the piston to be deeper into the caliper than in the recent past. It isn’t necessary to rebuild the calipers when replacing brake pads, but it is a good time to do it, along with a brake disk replacement .
Clean the outer surfaces of the brake caliper as completely as possible removing all traces of dirt and cleaning fluid. Clamp the piston to the mounting half of the housing, driving it back into the housing to make some room for the other piston to move out, and have an assistant use gentle pressure to push on the brake pedal. The rim side piston should come free, sometimes with a pop, so make sure you have something to catch the perished brake fluid. Alternatively, the piston just might come out far enough for you to grab it and twist it free. Remove the old seal from the inner bore with a blunt tool being careful not to damage the bore or the seal groove. Remove the old dust retainer and dust seal by prying with a screwdriver. The new seal kit will come with fluid seals and dust seals (which have a split lip on the face that goes towards the piston) and new dust seal retainers. Inspect the pistons for smooth surfaces and discard any that are suspect. New kits can also be purchased with new pistons, so you may want to be prepared to replace the pistons.
Clean the bores with brake fluid and a lint free cloth. Any corrosion that is present should be gently removed, as long as it is not in the sealing groove area of the bore. Lubricate the brake seal and groove with a brake seal installation grease such as Sil-Glyde® Brake Lubricant from AGS Company. (NAPA carries its own house brand of the same lubricant.) Coat the piston sides with Brake Lubricant. Insert the seal with your fingers working around until it is fully seated. Open the bleed screw one turn and insert the piston with the cut-away portion towards the hub. With the piston perfectly square to the bore, push the piston in until about 5/16 of an inch is protruding. With more brake seal installation grease, install the dust seal into the dust seal retainer and both around the piston. The retainer is fairly thin and will distort easily if forced so make sure you have it lined up properly and then seat the piston, seal and retainer evenly with gentle pressure.
Clamp the new piston on the rim side in place and tighten the bleeder screw. Now you can do the same process on the mount side piston, remembering to loosen the bleed screw and possibly the hose when pushing the mount side piston into place. If you are fitting new brake disks, this is the time to replace those as well. Once both pistons and seals are in place, mount the caliper to the swivel axle (remember the locking tabs) and fit the new pads (with brake grease on the piston face to reduce the chance of brake squeal), retainers and split pins, making sure the new pads move freely in their recesses. Bleed your system as you would normally and you’re done. There is a chance that you will encounter corrosion that is deep into the bore and that the calipers are no longer serviceable. If that is the case, you will need to purchase new calipers, these will come with pistons and seals installed.
This cross section of the brake caliper from the Workshop Manual which shows the caliper, pistons, seals and dust seal retainer. It may be of some help to visualize the process undertaken.
If your suspension and brakes are undergoing a full repair, it is advisable to strip away the old finish and rust, repair, weld and other wise refurbish the suspension parts as needed, and respray the paint. I use Rustoleum 7777 Satin Black for this purpose, but any satin black will do. You can use a rattle can or even cut liquid pints with 10-15% acetone and spray from your trusty paint gun, but after you’re done the brake calipers are going to look tatty. Most auto parts stores carry purpose made caliper paint in a variety of colors designed to tolerate the heat generated by the disk brakes. If this is your process, you are going to have to proceed a bit differently than just described, because the calipers are going to be coming completely off the car.
Removing the calipers and refurbishing them on the bench allows for careful refinishing of the exterior surface, but it also means that you will probably need to remove both pistons without benefit of having one piston in place to hold the pressure as you remove the other. This can be complicated because once both pistons are loose, and partially withdrawn, it is impossible to fully withdraw either. The approach I use on the bench is to work back and forth with clamps and hydraulic pressure to exercise the piston as much as possible so both pistons eventually move easily under pressure. Adding Brake Installation Grease to the process helps. Keep your fingers clear of any moving parts, and use brake fluid in the caliper, and judicious use of air pressure to help things along. If you decide in advance to replace the pistons force can be exerted on the exposed parts of the pistons.
Once the caliper pistons are withdrawn, you can remove the fluid seals in the bores and the dust seal retainers and dust seals as described before. The calipers are then cleaned inside and out, but take care with the use of petroleum based cleaners as there are internal seals between the caliper halves that should not be exposed to those products. The use of brake cleaning solvent on the outside of the caliper is acceptable with some care, when you are finished, wipe away the excess. Now carefully inspect the caliper bodies, pistons and other parts for corrosion, dirt and wear. This is the time to decide if rebuilding the caliper is practical, but from experience, except for the worst cases of corrosion, it typically is. Otherwise, purchase new complete calipers.
If the calipers pass muster, mask off the bores by stuffing paper towels in them, and put on several thin coats of your favorite color of the caliper paint mentioned previously. From here it is a simple task to install the seals, pistons, retainers and pads as described before for the process on the car. The only difference is that you can do both sides at a time, and once you have the dust seal retainers in place, you can push both pistons all the way in and install the brake pads, retainers and split pins.
With the calipers complete, and assembled, fitting new brake disks is recommended. Bolt the calipers to the car using the tabbed washers brackets; assure that the bleed nipple is in the upper port and fit the brake hose to the lower port. Note that the brake hose has to be loose from the cross member bracket before it can be turned into place. Add fluid to the brake reservoir and bleed the brakes according to your normal practice. Rebuilding the brake calipers, whether on the car or off can be done safely andis quite satisfying. As usual, if this is the first time you have tackled this project on your car, make sure you look over the workshop manual and possibly enlist the assistance of a trusted mentor. There is nothing inherently dangerous about working on the brake system of your MGB, but make sure you are fully prepared for the task before starting.