The brake pedal uses leverage to apply a strong mechanical force to the brake master cylinder piston. This is the first mechanical advantage in the brake system. The goal is to build tremendous pressure at the brake calipers or wheel cylinders without requiring too much legwork. A vacuum assist (sometimes hydraulic) booster is typically used to further multiply the force available using engine vacuum as an assisting force.
Drum brakes are fast disappearing from new vehicles, but many older vehicles had drums on the rear and sometimes on the front. A drum brake system consists of four major components on the axle: the supporting backing plate, hydraulic wheel cylinder, brake shoes, and the brake drum itself.
Disc brakes do not retain water as do drum brakes, and dry much quicker. Fine gravel and sand can also get stuck inside drums and wreak havoc on the brake shoes.
There is one advantage of drum brakes over disc brakes. Self-energizing drum brakes are actually designed to pull the drum brake shoe into the drum when the brakes are applied. This offers the driver further mechanical advantage. Power assist master cylinders are largely used for disc brakes that don’t offer this type of mechanical assistance.
Aside from leaking wheel cylinders, drum brake performance can suffer from worn shoes or drums that have been worn to an out-of-spec inside diameter. Chattering or surging may be due to drums that are old and have been worn out-of-round. Sticking or dragging rear brakes, or brakes that seem to get excessively hot may suffer from sticky wheel cylinders or broken/damaged retracting springs. Modern drum brakes are self-adjusting through a star-wheel screw mechanism at one end of the brake shoes. However, over time, the mechanism may start to stick and cease its adjustment function.
Disc brakes were first tried on vehicles over 100 years ago with limited success, then started to appear on post-WWII cars as a result of designs that were used on airplanes. Discs then replaced many front drum brakes in the ‘70s, and in the last decade or so discs have become common on rear axles.
There are three varieties of disc brake calipers: fixed calipers, floating calipers, and sliding calipers. A fixed caliper has one or more pistons, on each side of the rotor. The caliper is solidly bolted to the steering knuckle or other caliper mount. Pistons move equally from each side of the rotor to apply pressure directly on both sides of the rotor.
In contrast, a floating caliper has a single large piston (or two smaller ones) located on the inboard side of the caliper. Since force must be applied equally to both sides of the rotor, the floating caliper must be able to actuate both pads. The caliper is bolted to the mount with special pins, and the caliper slides back and forth on sleeves or bushings. The inboard pad sits directly on the piston. The outboard pad sits within the caliper frame. When the brakes are applied, the piston moves outward pushing the inboard pad into the rotor. Simultaneously, the force exerted backward from the piston onto the caliper frame causes the frame to move inboard, bringing the outboard pad into contact with the rotor. In this way, both pads press on the rotor.
A sliding caliper operates in much the same way as a floating caliper. Instead of using a set of pins, a sliding caliper rides in a set of machined guides. The guides allow for the lateral movement of the caliper.
Disc Brake Troubleshooting – Leave it to us
Disc brake calipers can leak which would require replacement. Once upon a time, you could rebuilt calipers however it has become more cost effective to simply replace them. Another failure of disc calipers occurs when a sticking caliper piston does not retract a bit once the brake pedal is released. This leaves one or both pads in contact with the rotor where they continue to drag on the rotor. This results in excessive rotor heat and premature pad wear. Uneven pad wear may also be the result of a caliper that is not floating freely on its pins or slides. When this occurs, most of the braking force is coming from just the piston side of the caliper. The other pad is doing little work, and braking performance suffers. Cleaning and lubricating the pins or slides should cure the problem. Uneven pad wear on a fixed caliper could also signal that a piston is sticking in its bore.