A sticking auto brake caliper is more than just an annoyance. If ignored, it can lead to dangerous driving conditions and cause serious damage to other brake system components. Even though different problems in the brake system can create similar side effects, there are ways to determine if it is the caliper piston sticking in your vehicle. Mechanics used to recondition the calipers when a problem occurred, but with current hourly labour rates up to and over triple digits, it's now more feasible to replace the damaged caliper with a remanufactured one.
Test drive the vehicle. A sticking or dragging caliper will not allow the brake pad to disengage from the surface of the brake rotor. Not only does this cause excessive premature brake pad and rotor wear, but the vehicle will be literally driving with the brakes slightly applied all the time. A telltale sign of a severely sticking caliper piston is the vehicle pulling to one side when driving. If you constantly have to hold the steering wheel in place, it may not have anything to do with steering or wheel alignment. This can also stress the transmission of the vehicle.
Test the heat coming off the wheels after the test drive by placing your hand near the wheel without touching it. Sticking calipers will cause brake pads to constantly drag on the rotors of the braking system and this will create a tremendous amount of heat. The heat will then transfer to the wheel/rim of the tire. Be careful not to directly touch the tire rim as the heat can cause severe burns.
Lift the vehicle with a floor jack and secure it on jack stands to visually inspect the difference in brake pad wear from one side to the other. While pads wearing more on one side may simply mean that pads are improperly lubricated and stuck in the bridge of a caliper, it could also indicate the onset of a sticking caliper piston.
Remove the lug nuts with the lug wrench and remove the wheels.
Place the top of a large C-clamp over the inboard caliper housing and the bottom of the clamp onto the outboard pad and tighten the clamp to ascertain if the caliper piston is sticking or stuck. A properly functioning caliper will allow you to tighten the C-clamp and compress the caliper piston. A sticking caliper is going to be very difficult, if not impossible, to compress. Compare one caliper on the same axle to the other. Be careful when applying this procedure to rear calipers on certain vehicles as some caliper pistons (on some imports) require a screw-in caliper piston and will not compress by squeezing with a C-clamp.
Remove the caliper mounting bolts and attach the caliper to the chassis of the vehicle with mechanics wire. Do not allow the caliper to dangle precariously from the rubber brake hose.
Remove the pads from the bridge of the caliper, but mark them or position them after removal in order to remember how to replace them in their exact, original position. If the pads are stuck in the bridge and need to be forced or pried out with a screwdriver, this may be the root of the problem and not the caliper piston sticking. Clean the caliper bridge surface using a wire brush or an angled die grinder and a reconditioning disk. Remove and clean the metal hardware and then replace it. Apply a liberal coat of brake lubricant to the pad contact points of the hardware and the bridge. This could revive the braking system if the problem has not done too much damage to the brake pads already.
Take an overall measurement of each brake pad's thickness using a tire tread depth gauge or a micrometer. Take several measurements of each pad in different locations and compare the measurements to the other pads. While there will be some variance in wear, perhaps even on the same pad, an obvious visual variance in pad thickness may be an indication of a sticking caliper. A front brake pad wearing down below 4/32 of an inch is getting close to needing replacement. A rear brake pad wearing down below 3/32 is getting close to needing replacement. Rear disc brakes do not work as hard as the front pair, so less friction material on the pad is more acceptable.
Sticking brake calipers or brake pads stuck in the bridge should be corrected immediately. Either one of these brake system problems can cause damage to other braking components on the vehicle and stress the engine and transmission.
After reassembling the brake components, remember to reseat the pads to the rotors by pumping the foot brake pedal. Any time a caliper (or other hydraulic braking component) is replaced, be sure to properly bleed air out of the brake system.