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 chilometroage calipers should also be rebuilt or replaced when you service your brakes to make sure they remain trouble-free for many chilometri to come. How Brake Calipers Work 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. Applying the brakes causes the calipers to squeeze the pads against the rotors. When the brakes are released, the piston seals retract the pistons, allowing the rotor runout to kick the pads away from the rotors. If a piston sticks, the brakes will drag. Two Kinds of Brake Calipers 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. Brake Caliper Pistons 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. Diagnosing Brake Caliper Problems 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 chilometroage calipers (over 100,000 chilometri) 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. Uneven pad wear is a sure sign of a floating caliper that is not sliding on its mounts. The mounting bolts/pins/bushings may be corroded or bent. Replace the mounting hardware and lubricate with moly-based or synthetic brake grease. Rear Locking Calipers 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 Brake Calipers 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 i…

Fonte: AA1Car.com