Brake caliper problems include fluid leaks caused by worn piston seals, uneven braking (pulling to one side when the brakes are applied), which can be caused by a sticking caliper piston or the caliper hanging up on its slides/bushings, and dragging (usually due to a sticky piston that won't release the brake). If you are having a brake caliper problem, you will have to rebuild or replace your calipers. Replacing your calipers is usually the best course of action because it is faster, easier and more reliable than attempting to rebuild an old caliper. High mileage calipers should also be rebuilt or replaced when you service your brakes to make sure they remain trouble-free for many miles to come.
The caliper is the part of a disc brake that squeezes the pads against both side of the rotor when the brake is applied. Hydraulic pressure generated by the driver's foot pressing on the brake pedal goes from the master cylinder to each brake caliper. Fluid pressure inside the caliper pushes one or more pistons outward to apply the brake.
When the brake pedal is released, hydraulic pressure drops and the calipers loosen their grip. Square cut seals around the caliper pistons deform when the pistons move, so when pressure is released the seals want to return back to their original shape. This helps retract the pistons slightly,allowing the pads to kick back away from the rotors. This eliminates the need for return springs that are necessary in drum brakes.
Floating calipers (also called sliding calipers) are mounted on slides or bushings that allow the caliper to move in and out sideways when the brakes are applied and released. Floating calipers typically have a single piston located on the inboard side of the caliper.
When the brakes are applied on a vehicle with floating calipers, the pistons move outward and push the inner pads against the rotor. This forces the calipers to slide inward slightly and pull the outer pads up against the rotor to apply the brake. When the pedal is released, the calipers slide out slightly as the pads are kicked away and retract from the rotors.
Fixed calipers, by comparison, are rigidly mounted over the rotor and do not slide or move when the brakes are applied. This type of design requires pistons on both sides of the rotor to apply the pads.
With both types of calipers, the pistons will move further and further out in their bores as the pads wear. This will cause a corresponding drop in the fluid level in the master cylinder reservoir.
Floating calipers may have one or more pistons, while fixed calipers usually have multiple pistons. The pistons in floating calipers are located on the inner side of the caliper while those in fixed calipers are located on both sides of the rotor (inner and outer).
Single piston calipers usually have a large diameter piston while multi-piston calipers have smaller diameter pistons. One of the advantage of using more than one piston is that a longer brake pad can be used in the caliper housing. Also, multiple pistons spread the force across the back of the pad, allowing more even brake application (less pad distortion). The disadvantage of a multi-piston caliper is that if one piston sticks, the pads will not be pressed evenly against the rotor when the brakes are applied. This can cause uneven pad wear and reduced braking effectiveness (increased stopping distance).
Caliper pistons may be aluminum, steel or phenolic (plastic). Each type of piston has certain advantages and disadvantages. Aluminum pistons are lightweight but vulnerable to corrosion. Aluminum also transmits heat from the pads to the brake fluid inside the caliper, which can increase the risk of fluid boil and brake fade if the brakes get too hot. Steel pistons are the strongest type of piston, but like aluminum are vulnerable to corrosion (rust). Steel pistons are usually plated with nickel or chrome to improve their resistance to rusting. Steel pistons also transmit heat from the pads to the brake fluid in the calipers. If an aluminum or steel piston corrodes, it may stick in its bore, preventing the caliper from applying the brakes or from releasing the brake once it has been applied. Phenolic pistons are made from cast resin and are lightweight. Phenolic pistons will not corrode but they can absorb moisture and swell, causing them to stick in their bore if the manufacturing clearances are not correct. Phenolic pistons do not conduct heat like metal pistons, which reduces heat transfer from the pads to the fluid in the calipers and reduces the risk of fluid boil and brake fade when the brakes get hot.
WARNING: If you are rebuilding calipers that have steel or aluminum pistons, do NOT sand the pistons to remove rust or corrosion. Sanding steel pistons will remove the protective plating that helps prevent rust. Sanding aluminum pistons will remove the anodized coating that prevents corrosion. Replace rusty steel pistons or corroded aluminum pistons with new ones.
Leaks are the most common type of problem you will find with brake calipers. To check for leaks, inspect the area around the caliper pistons. The outer dust seal may be cracked or torn, but if there is no sign of fluid the piston seals are probably okay. But if you see any fluid seepage, the piston seals are leaking.
Also check the bleeder screws on each caliper. If the bleeder screw is not seated tightly, it may leak fluid.
If a caliper is leaking brake fluid, it must be rebuilt or replaced. Leaks are dangerous and must not be ignored because ANY loss of fluid may result in brake failure. Leaks often develop over time as seals age and harden. Corrosion in the piston bores or on the pistons is what usually causes the seals to wear and leak. Rubber seals also lose elasticity with age, and may harden and crack, resulting in a leak.
As a rule, high mileage calipers (over 100,000 miles) should usually be rebuilt or replaced when the brake pads are replaced - whether or not the calipers are leaking. The original piston seals won't last forever, so why take a chance?
The caliper castings should also be closely inspected for any signs of damage or cracks. Replace the caliper if it has cracks or damage.
If the inner and outer brake pads are worn unevenly, or the caliper has been sticking, inspect the condition of the caliper slides, pins and bushings. Rust and wear can cause a floating caliper to stick. The old caliper slides,bushings and mounting hardware should all be replaced with new parts to assure proper operation. The slides and bushings on floating calipers should also be lubricated with a moly-based or synthetic high temperature brake grease so the calipers will move in and out freely on their slides/bushings.
If the slides or bushings on a floating caliper are rusty, worn or damaged, the caliper may stick causing the pads to drag and wear unevenly. A "sticky" or "frozen" caliper can also cause the brake to drag if the caliper does not allow enough movement to release the brake.
Fixed calipers don't suffer from this type of problem because the position of the caliper is fixed and the caliper does not move in or out. But both fixed and floating calipers can sometimes "freeze up" or lock if a piston becomes jammed or stuck in its bore due to corrosion or swelling. When a piston sticks, one of two things can happen: the brake may not apply or more likely, it may not release. The uneven braking that results from a sticking piston will produce a hard pull toward the "good" side when the brakes are applied, and/or a steering pull towards the "bad" side if the caliper is sticking and dragging.
On vehicles with four wheel disc brakes, the rear calipers are often locking calipers that apply the parking brake. A mechanism inside the caliper pushes or moves the piston out to apply the brake when the parking brake cable is pulled. The piston may screw in and out, and is self-adjusting to maintain proper pad-to-rotor clearance. Corrosion in the piston adjuster or parking brake mechanism can cause the piston to stock, preventing the parking brake from applying or releasing.
Rear locking calipers can be difficult to rebuild, so if the caliper is worn or sticking it is often best to replace it with a new or remanufactured caliper.
Rebuilding a brake caliper requires disassembling the caliper (removing the piston(s) from their bores), cleaning and inspecting the caliper bores and pistons, and replacing the old piston seals and dust boots with new parts. If a piston is corroded, rough or pitted, it must be replaced otherwise it may stick in its bore and/or damage the new seals. If the piston bore surface in the caliper is rough, pitted or corroded, you should replace the caliper.
The piston bores in cast iron calipers can be sanded or honed to make them smooth again, but too much sanding may increase the inside diameter of the bore or lease an uneven surface, which may cause the piston seal to leak or not retract the piston properly when the caliper is put back on your vehicle. If the caliper is aluminum, sanding the bore is not a good idea because it will remove the protective hard anodized surface that prevents wear and corrosion. This could lead to leaks later on.
When a used caliper is remanufactured, the piston bore may be bored out and sleeved to restore a smooth surface. Sleeving also restores the proper inside diameter of the bore so the seal will seal and retract the piston properly. If you are buying a remanufactured caliper for your vehicle, ask your parts supplier if this has been done on the rebuilt caliper. If not, it may be a cheap rebuild that won't last.
Removing the calipers is a fairly simple job on most vehicles, but on some late model cars that "pre-energize" the brakes to reducing braking times, the brake system may have to be deactivated with a scan tool (or by removing a brake system fuse) prior to working on the brakes. Always refer to the vehicle service literature to see what precautions and procedures should be followed when working on your brakes.
Most calipers are held in place by a pair of bolts or pins. Once the bolts or pins have been removed, the caliper can be lifted up and away from the rotor. You can then remove the inner and outer brake pads and insect the caliper.
A caliper should be replaced if it is damaged, cracked, badly corroded, worn or leaking. You may also have to replace a caliper if the bleeder screw is frozen or has broken off. Broken bleeder screws can sometimes be removed with an "easy-out" tool, or the old bleeder screw can be drilled out and the hole retapped to accept a new bleeder screw. Make sure no debris remains inside the piston bore after repairing the broken bleeder screw.
If a piston is frozen or stuck in its bore, replace the caliper. Don't waste your time trying to rebuild it.
WARNING: Trying to remove a piston from a caliper housing by blowing compressed air through the bleeder screw is dangerous! The piston may suddenly pop out of its hole with great force and pinch your fingers if they are in the way!
Calipers are usually rebuilt or replaced in pairs depending on their condition and mileage. But if only one caliper needs replacing because of a leak or problem, the replacement caliper should have the same type of piston (steel or phenolic) as the one on the opposite side, as well as the same type of friction material (ceramic or semi-metallic). Mismatches in friction materials or type of piston side-to-side can cause uneven braking.
NOTE: Remanufactured calipers may or may not have the same type of pistons (steel or phenolic) as the original calipers. The type of piston really does not matter as long as the pistons on the right and left calipers are the same. Different caliper pistons (or types of pads) on either side of a vehicle may cause uneven braking. If you are buying "loaded" (pre-assembled) calipers, they come complete with new pads and mounting hardware. Loaded Calipers are faster and easier to install than bare calipers. Loaded calipers should be replaced in pairs (right and left side).
If you are rebuilding your old calipers yourself, use silicone brake grease (never ordinary chassis grease) to lubricate the pistons and seals. Petroleum-based greases will contaminate the brake fluid and may cause problems with the seals.
WARNING: When working on the brakes, never allow a caliper hang by its hose as this may damage the hose. Support the weight of the caliper with a piece of wire or rest the caliper on the suspension.
Replacing a caliper requires opening the brake lines, so you will also need brake fluid. The brake lines and calipers must be bled after the new calipers have been installed to remove air from the system (air can cause a soft pedal and increased pedal travel). You will need a brake bleeder tool to bleed the brake lines and calipers.
Replace any rubber brake hoses or steel brake lines that are damaged or leaking.
When installing the caliper back on your vehicle, use a torque wrench to tighten the caliper mounting bolts to specifications.