What Are Brake Callipers and How Do They Work?
Brake callipers are essential to your car’s ability to stop and are arguably one of the most important automobile brake parts. Most cars today have disc brakes, at least for the front wheels, anyway. But a lot of cars and trucks are now using disc brakes in the rear, too. In a disc-braking system the car’s wheels are attached to metal discs, or rotors, that spin along with the wheels. The job of the calliper is to slow the car’s wheels by creating friction with the rotors.
The brake calliper fits over the rotor like a clamp. Inside each calliper is a pair of metal plates bonded with friction material — these are called brake pads. The outboard brake pads are on the outside of the rotors (toward the curb) and the inboard brake pads on the inside (toward the vehicle). When you step on the brake, brake fluid from the master cylinder creates hydraulic pressure on one or more pistons in the brake calliper, forcing the pads against the rotor. The brake pads have high-friction surfaces and serve to slow the rotor down or even bring it to a complete halt. When the rotor slows or stops, so does the wheel, because they’re attached to one another.
Older cars and trucks used drum brakes, where the motion of the wheels is slowed by friction between a rotating drum and brake shoes mounted inside the drum. This friction caused heat and gases to build up inside the drum, which often resulted in a loss of braking power known as brake fade. Because the brake pads in disc brake systems are external to the disc rather than contained within a drum, they are more easily ventilated and heat doesn’t tend to build up quite as fast. For this reason, drum brakes have been largely replaced in modern vehicles by disc brakes; however, some less expensive cars still use drum brakes for the rear wheels, where less stopping power is required.
here are two main types of callipers: floating (or sliding) callipers and fixed callipers. Floating callipers move in and out relative to the rotor and have one or two pistons only on the inboard side of the rotor. This piston pushes the entire calliper when the brakes are applied, creating friction from the brake pads on both sides of the rotor. Fixed callipers, as the name implies, don’t move, but rather have pistons arranged on opposing sides of the rotor. Fixed callipers are generally preferred for their performance, but are more expensive than the floating kind. Some high-performance fixed callipers have two or more pairs of pistons (or “pots”) arranged on each side of the rotor — some have as many as six pairs total.
Special tools are useful when working with brake callipers, especially when replacing the brake pads. We’ll talk about that in the next section, and then discuss the different types of brake callipers available for different types of vehicles.
How to Tell If Your Brake Callipers Need Replacing
- Open the hood and check the fluid level of the master cylinder. A leaking piston, bleeder or brake hose will drain the reserve fluid from the master cylinder eventually. If necessary, top the master cylinder off with clean brake fluid.
- Test-drive the vehicle to begin diagnosing the callipers. Choose a parking lot or low-traffic road if possible. Before taking off, be sure you have a firm brake pedal. If the brake pedal fades to the floor with a poor braking response, do not test drive the vehicle.
- Bring the vehicle up to 30 mph and then apply the brakes firmly. Determine whether the vehicle pulls to the right or the left when the brakes are applied. This would be a good indication that a calliper piston is sticking and only one side of the brakes are working properly.
- Drive the vehicle for 10 to 15 minutes applying the brakes often. Bring the vehicle back to the area where you plan on disassembling it for further diagnosis. Before beginning, place you bare hand near each wheel that employs callipers. Do not touch the wheel because a stuck piston will heat the brakes and nearby components up severely and you could burn your hand. Burning brakes will give off both intense heat and a pungent burning odour. If a wheel is indeed extremely hot, allow it to cool down before proceeding.
- Lift the axle of the vehicle that contains the suspect calliper. Use the jack to lift it and be sure to place it safely onto jack stands.
- Remove the wheel using a lug wrench to remove the wheel nuts.
- Remove the two calliper guide bolts using a ratchet and a suitable socket.
- Pry the calliper off of the rotor with a small pry bar. Some callipers will contain pads attached to the calliper while others will leave the pads behind in the calliper anchor.
- Inspect the calliper for any visible signs of fluid leaks. Leaking brake fluid will be very obvious. Thoroughly inspect the piston area, the bleeder screw and the brake hose connection. Also inspect the rubber protective boot surrounding the pistons. Tears or rips in the boot will not necessarily mean the calliper is defective, but it will compromise the pistons eventually. While you’re at it, inspect the brake hose for any visible signs of cracking or rips in the rubber.
- Hang the calliper to the suspension using a calliper hook or a wire hanger. Compress the piston using a C-clamp or a calliper piston reset tool. Some rear disc-brake callipers will require the reset tool to screw the piston into the bore while most all front callipers will require compressing the piston into the bore with a C-clamp. Compress it slowly. If the piston does not bottom out in the bore or resists compression, the piston is bad and the calliper needs to be rebuilt or replaced.
- Check the slides and the pads next. Some vehicles are notorious for sticking calliper slides or pads that get stuck in the bridge. These two symptoms will give off similar evidence that the calliper is faulty. Removing the sticking slides or the stuck pads and re-lubricating the anchor or slides are all that is usually required and the calliper can be reused.
h/t to itstillruns.com for the great info!