How Brake Calipers Work

supercar brake system featuring dual fixed brake calipers
Supercar, Commuter, or Truck, How do Brake Calipers Work?.

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Like a decent remote-control car, your vehicle has a few basic functions: move forwards and backwards, turn left and right, and stop. Of course, stopping a one-ton-plus car requires more than simply letting off the throttle, and slamming it in reverse might destroy the transmission. Your car’s brake system has come a long way since Bertha Benz, Karl Benz’s wife, invented brake pads in 1886.

There are basically two kinds of brake systems: disc brakes and drum brakes. Drum brakes are an older technology, not as powerful nor as efficient, but still in use in some applications because they’re cheaper to produce and pretty good for rear brakes in most vehicles. Disc brakes are a newer technology, better than drum brakes in every way, but also more expensive to produce and maintain.

What Is a Brake Caliper?

new brake pads and new brake calipers
What Exactly is a Brake Caliper, Anyway?.

The disc brake system is made up of a few basic parts, including the brake caliper, brake rotor, brake pads, and various shims, springs, and clips to hold the pads. The brake rotor, or brake disc, mounts between the wheel and axle hub, rotating with the axle and wheel. The brake caliper is fixed to the steering or suspension knuckle. Gripping the rotor, the brake caliper can reduce the speed of the wheel to the speed of the steering or suspension, that is, zero – more on that in a minute.

Brake calipers come in two basic types, fixed brake calipers and floating brake calipers. Fixed brake calipers are bolted directly to the knuckle and all moving parts are internal. Inside the block of a fixed brake caliper, two to four pairs of pistons compress the brake pads, which slide on pins, from both sides. Floating brake calipers are not mounted directly to the knuckle, but to a “cage.” The cage carries the brake pads, usually on sliding rails, and the brake caliper slides over them, mounted with sliding bolts. Inside a floating brake caliper are one or two pistons on the inboard side.

How Do Brake Calipers Work?

basic diagram of disc brake caliper operation, from brake pedal and master cylinder to brake caliper and brake rotor
A Basic Diagram of Brake Caliper Function.

At their most basic, brake calipers are force-multiplication devices. Step on the brake pedal and a small piston compresses brake fluid in the master cylinder. Because brake fluid doesn’t compress, this force is transmitted instantly to the brake calipers. Inside the brake caliper, large pistons multiply the force exerted, pushing the brake pads into the brake rotor.

In the case of fixed brake calipers, the pistons compress from both sides. In the case of floating brake calipers, the piston pushes first on the inboard brake pad, pushing the caliper away from the rotor, causing the outboard brake pad to contact the rotor. The caliper slides allow for this movement.

How Do Brake Calipers Fail?

comparing worn and new brake pads
A Sticking Brake Caliper Slide or Brake Pad Could Lead to Accelerated Wear.

Fixed brake calipers are more expensive, but also more efficient and more reliable, while floating brake calipers are sufficiently reliable to offset the cheaper production costs. Still, brake calipers can fail in a few ways. Here are some common brake caliper failures and how to fix them.

  • Stuck Caliper Slider: On floating calipers, caliper sliders are the weakest link and cause many problems. Accelerated wear on the inboard pad is common enough, but a sticking slider exacerbates the problem. If the caliper slider can’t move freely, it could lead to more wear on the inboard pad, dragging of the outboard pad, or reduced braking efficiency, no braking at all on the outboard pad. If one of the sliders is sticking, this might lead to a spongy brake pedal feeling, as the brake caliper flexes trying to make full contact with the rotor.
    • Clean out caliper slides with a drill or wire brush and lubricate with heavy silicone grease. Replace dried-out or torn boots to prevent water entry and contamination.
  • Leaking Caliper Piston: In fixed or floating calipers, each piston has a square rubber seal, which holds brake pressure and pulls the piston back slightly on release. An external rubber boot keeps water and dust out of the piston bore. Due to age or poor installation practices, torn dust boots could allow water and dust into the piston bore, accelerating corrosion. If the piston seal passes over this corrosion, it will likely be damaged and leak.
    • Check that dust boots are sealed, intact, and dry. Do not use silicone grease in this area, as it is incompatible with brake fluid. If rebuilding brake calipers, be sure to hone piston bores, clean and dry everything, replace all seals, and install all boots. Use fresh brake fluid to lubricate seals for installation.
  • Sticking Brake Pad: There are tight clearances between the brake pads and retaining hardware. This keeps things from bouncing around and making noise. Reduced clearances also make for more efficient braking. Over time, corrosion can reduce this clearance, causing the brake pad to drag or stick. A sticking brake pad may drag on the brake rotor or not move to engage the rotor, leading to accelerated wear, overheating, and inefficient braking. A sticking brake pad can mimic a stuck slider in the way it feels and acts.
    • When assembling brakes, make sure to clean all corrosion and eliminate dust and grime buildup. Don’t forget to clean behind clips and springs. A tiny amount of anti-seize will protect rails and pins from corrosion, though paint would be a better option.

Though only comprising a few parts of your car, brake calipers are one of the most critical, enabling controlled braking in varying situations. Knowing how they work and how they move also helps you to make informed decisions regarding their maintenance and repair, whether you consult a professional or do-it-yourself. When it comes to brake calipers, always double-check everything, and be wary of unusual noises before they cause an unsafe condition.