January 2000

Brake Caliper Tune-up By Steve Grosekemper When most people think of getting their car in peak performance, the item they think of first is usually the engine. The second thing they should be thinking about is the brakes! After all, you will have to stop that car eventually. We are going to cover the operation and maintenance of the brake caliper. This is the place where most braking problems can be found when all else appears to be normal. Porsche braking systems are one of the best systems in the world when everything is just right. The way a brake caliper works is quite simple. When you step on the brake pedal, you pressurize brake fluid that travels through brake lines and hoses to the brake caliper. This pressurized fluid pushes a piston and subsequently a brake pad and clamps down on a brake rotor, causing the wheel to stop. The relationship between the pistons, brake pads, and rotors is not as simple as it seems. The caliper must load the brake on the trailing edge of the pad. This is done so the pad bites into the rotor evenly for more stopping power. Think of it as if you were moving a 100-pound bag of sand across a dirt lot. If you pull it behind you, the front of the bag will be up and skim across the top of the sand (greatest load to the rear), leaving an even trail behind you. If you get behind the bag and try to push it (greatest load to the front), you will cause the front of the bag to dig into the dirt and create a hole. In the case of the brakes, the leading edge of the pad would then see greatly increased uneven wear. The way the caliper achieves this task varies with design. For the purpose of our discussion, there are three basic types of calipers: single piston, 4-piston, and 4-piston/offset diameter. The first type, single piston fixed, includes 356, 911, 912, and 914 up to 1989 model years. These cars have one piston on each side, pushing on the rotor. Early 944s and 928s have one piston per caliper that clamps down on each brake pad (single piston floating). The pistons are machined with a 20-degree notch in them to keep even pressure on the brake pad as it moves across the surface of the rotor. If the piston rotates out of position, the result can range from poor brake performance and pulling to poor brake pad wear. The remedy is to check the piston position with special tool (P84) with the brake pads removed. If you don't have this tool use Figure #1 as a template and fabricate one out of a metal or plastic. Use Figure 2 to see the correlation between rotor direction and piston position. This is also a good time to inspect and replace dust boots which may be deteriorated. The second type of caliper is the standard 4-piston. Models using this caliper are '78-'86 930s, and early 944 Turbos. They can be visually distinguished by external cooling fins. This caliper has 4 pistons of equal diameter with a setback surface on 50% of the piston shoulder. These pistons do not sit in the caliper at a 20-degree position. The edges of the shoulder on the piston must be parallel with the brake pad guide surface. The setback surface must face the leading edge of the rotor. (See Figures 3 and Figures 4 for front and rear caliper illustrations.) The final caliper type is the 4-piston offset type. This caliper is visually distinguished by a smooth exterior surface. It has 4 pistons, as in the earlier versions, with one major design difference. The pistons on the leading edge of the caliper are of a smaller diameter. This smaller diameter causes a lower clamping pressure and acts in the same manner as the setback surface on the earlier style caliper. This caliper has two sub types: one with a larger piston with a setback surface as discussed earlier (see Figure 5). The other has two different piston sizes with no setback surface, thereby eliminating the need to check caliper piston position. As a rule, when correcting any caliper piston positioning the brake pads should be replaced. The reason is because if a piston is out of position, the brake pad surface will no longer be parallel with the pad backing. This will cause uneven pad application and possible piston binding. Good Luck!