In disc brake calipers, the piston is sealed by a stationary square section sealing ring, also called a square cut O-ring FIGURE 32-11. This O-ring has a square cross-section and is fitted in a machined groove in the caliper. The O-ring is compressed between the piston and caliper housing, creating a positive seal to keep the high-pressure brake fluid from leaking out. It also prevents air from being drawn into the system when a low-pressure situation is created in the hydraulic system when the brake pedal is released quickly.
AB
FIGURE 32-11
O-rings. A. Square cut O-ring and O-ring cut to show square section. B. Square cut O-ring groove in caliper.
AB
FIGURE 32-12
Square cut O-ring. A. Square cut O-ring during brake application. B. Square cut O-ring during brake release.
When the brakes are applied, the piston moves outward, slightly deforming the O-ring seal FIGURE 32-12. When the brakes are released, the elasticity or flexibility of the seal causes it to return to its original shape. This action of the sealing ring retracts the piston to provide a small running clearance between the rotor and pads. As the brake pads wear, the piston needs to move outward a bit farther than the sealing ring can stretch or flex. The sealing ring is designed to allow the piston to slide through it in this situation, taking up the extra clearance and making the disc brakes self-adjusting.
Some calipers, sometimes called low-drag calipers, are designed to maintain a larger brake pad-to-rotor clearance by retracting the pistons a little bit farther. This is accomplished by modifying the sealing groove in the caliper so the outside of the groove is slightly angled toward the rotor FIGURE 32-13. This position allows the seal to flex a bit farther upon brake application and then retract the piston a greater distance. These systems use a “quick take-up” or “fast fill” master cylinder to maintain adequate brake pedal reserve height.
The primary sealing surface is the outside diameter of the piston. It is critical that this surface be smooth and free of pitting or rust; therefore, steel pistons are chrome plated. This gives the surface a hard, wear-resistant, and corrosion-resistant finish. Chrome can still rust, but it is much more corrosion resistant than steel. Another way manufacturers have dealt with the corrosion issue is by making pistons out of a phenolic resin. Pool balls also are made from phenolic resin, which is very dense when it hardens and does not corrode or rust, making for a good sealing surface in brake systems. Although the phenolic pistons themselves do not corrode, the cast iron bore of the caliper does corrode and rust and can therefore cause a phenolic piston to seize in the bore FIGURE 32-14.
FIGURE 32-13
Low-drag caliper.
FIGURE 32-14
Corroded caliper piston bore.
FIGURE 32-15
Heat transfer. A. Phenolic piston (slow heat transfer). B. Steel piston (fast heat transfer).
Phenolic pistons transfer heat slower than steel pistons FIGURE 32-15, which is a good thing because they transfer less heat from the brake pad to the brake fluid. This helps prevent boiling of the brake fluid in the caliper under heavy brake usage. Calipers with phenolic pistons are therefore less susceptible to brake failure from boiling brake fluid.
There is also a dust boot that seals the surfaces of the piston and caliper bore from outside dirt and moisture. This seal connects to both the piston and the caliper and must be expandable to allow the piston to move outward as the brake pads wear. It also must be free from cuts and holes; otherwise the piston and bore could corrode and cause the piston to bind in the bore, causing brake drag.
TECHNICIAN TIP
Since the force generated by the O-ring to retract the piston is fairly small, any corrosion or buildup on the piston or bore will cause the piston to stick and not retract. This holds the brakes in the applied position, causing brake drag, overheated brakes, and poor fuel economy. Technicians can identify this situation by using an infrared temperature gun to measure the temperature of each brake rotor after test-driving the vehicle. The temperatures should be approximately the same on each side. If they are not, suspect a stuck or binding caliper.
