In part one, we discussed brake rotors, pads and causes of brake shudder and noise. In part two we will look into brake calipers and their problems. (please click images for a closer view)
Fixed Mount brake Caliper
There are several designs of brake calipers in use. There are single, dual and multiple piston designs as well as sliding and fixed mount. With the fixed mount caliper there are pistons on each side of the rotor. The inboard and outboard pads are directly applied by the moving pistons.
A fixed mount caliper may have one or two pistons on each side. The more pistons used, the more even the pressure that is applied. For this reason multiple piston calipers are often used on sport models and heavy duty applications.
Fixed mount calipers give little trouble, but are more expensive. For this reason they are not very common. Far more common is the sliding mount brake caliper.
Sliding Mount Brake Caliper
With the sliding mount caliper there is a piston or pistons on only one side of the brake rotor. The piston is mounted in a sliding bracket that moves in the caliper mount. When the piston(s) apply the inboard pad, the bracket slides in reaction and applies the outboard pad.
The slides may be metal pins or other arrangements. It is very important that the slides are properly lubricated. This should be done at every brake service and more often if there are problems. The protective boots that seal the slides must also be kept in good working order.
If the slides seize, the inboard pad will apply and the outboard will not. If the slides bind, the outboard pad will not release when the brakes are released. In either case, rapid pad wear and rotor problems will result.
The pressure of applying only one pad will cause the brake rotor to warp. A dragging brake pad will cause excess heat and also warp the brake rotors. Any time brake rotors warp or brake pads wear quickly and unevenly, caliper slides should be carefully inspected and serviced.
Other Brake Caliper Problems
Brake calipers can also fail in other ways. The piston can stick in the bore or the seal can fail causing them to leak. When this happens they can be rebuilt or replaced. Some calipers use a phenolic piston. Phenolic pistons do a good job of resisting corrosion, isolating heat from the fluid and reducing noise.
Over time, the coating on the phenolic piston can also wear. When this happens fluid can cause the piston to swell and stick in the caliper bore. Sticking calipers will cause rapid pad wear, noise and a great deal of heat. Pads and rotors can be quickly ruined by a sticking brake caliper.
With metal piston calipers corrosion can cause the same type problems. Replacing brake fluid on a regular basis can help to prevent system corrosion and other problems.
Brake hoses also fail, sometimes with no outward signs. Many of the calipers we disassemble are seized because deteriorated hose material has caused the piston to stick. Age is the leading cause of brake hose failure. Please see our detailed topic on brake hoses for far more information.
Pads and rotors work as a set. When rotors are replaced, the brake pads should also be replaced, with new original equipment pads. It is also imperative to replace any missing clips, springs or shims.
Noise is vibration and vibration cannot be stopped. Instead, by properly lubricating the mounting points the pads can move without causing noise. Special high temperature brake caliper grease should be applied to all metal to metal contact points.
The mounting surface of the brake rotor must also be clean and true. A bent or damaged hub will result in repeated warping of brake rotors. Wheel lugs should be tightened with a torque wrench to the specified torque and in the proper pattern to avoid problems.
By observing a few precautions most brake noise and rotor warping problems can be avoided. When you have brake problems call AGCO, it’s the place to go.