I have been doing my own car work for quite a few years, but I have a few questions about disc brakes that always intrigued me. How is it that drum brakes require return springs for the brake shoes but disc brakes seem to have none? How often should rotors be replaced or machined? What about rebuilding or replacing the calipers? How does the emergency brake work when a car has disc brakes in the back? Is it hydraulic? I haven’t yet worked on a rear-disc car. – Pablo Morales, Gilroy, Calif.
Wow! I’ll do my best to squeeze answers to these great questions into such a short space. Disc brake pads are pressed against the rotor by the caliper piston(s), which floats in a rubber seal and expandable dust boot. As hydraulic pressure is applied to the back of the piston and it slides forward, the square-cut seal rolls just a bit, then snaps back into the original position when the pressure is released, bringing the piston back with it.
Imagine shaking a bottle of champagne where the cork bulges slightly without necessarily moving, then after a time, returns to its original shape. The piston will slide within the seal gradually as the pads wear thin, taking up vacated space, but during a typical brake application piston movement is very slight, due to the tiny clearance between the brake pads and rotor. Pad drag is negligible, due to the slight run-out in the rotor knocking them back slightly.
Brake rotors must have a smooth, clean and parallel surface finish and be of adequate thickness to absorb and release heat. Wear and refinishing (cutting or grinding the rotor surface to restore smoothness and parallelism) can result in a thin rotor, which is prone to warping during heavy use. When a rotor wears or is cut to its minimum specified thickness, it must be replaced. I prefer not to resurface a rotor, favoring thickness, unless notable surface flaws or warping (felt via a pulsating brake pedal) dictate the need.
Disc brake calipers are amazingly durable. If one flushes/renews the brake fluid perhaps every three to four years, the calipers should last the life of the vehicle. On rare occasions a caliper might begin to seep or leak soon after brake pad replacement. Installing new, thicker pads requires pushing the piston back into the caliper to a position it hasn’t seen in several years. Corrosion pitting on the piston, between the seal area and dust boot, might score the seal, leading to seepage or leakage. If there’s no evidence of seal seepage, and the vehicle has had periodic fluid changes, I’d be comfortable reusing the existing calipers.
Disc brakes pose an interesting challenge for the parking brake system. Some rear disc systems employ a tiny brake drum and shoes within the rotor while others use the more common lever and cam/screw arrangement to press the caliper piston forward. Sticking/binding of this mechanism can occur, causing poor parking brake operation.
When renewing rear disc brake pads one must be careful to rotate the caliper piston, using a special tool, rather than simply pushing it back into the caliper.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.