An air disc brake caliper is the housing and mechanism that presses the brake pads against a rotating brake disc (rotor) to slow the wheel. On trucks, tractor units, buses and trailers it takes its force from a compressed-air brake chamber, multiplies it with a lever and eccentric, and takes up clearance automatically as the pads wear. A faulty caliper usually shows up as dragging, heat and uneven pad wear.
In short:
- An air disc brake caliper passes the brake chamber force through the operating lever, bridge and tappets to the inner pad, and its housing, sliding on guide pins, pulls the outer pad onto the disc.
- The automatic adjuster keeps the running clearance roughly constant as the pads wear; if the clearance gets too small the brake drags, and if it gets too large the braking effect drops.
- Calipers stick mainly because water enters through torn boots or a leaking brake chamber, because guide pins rust, or because the adjuster is damaged.
- Boot, pin and bushing problems can often be fixed with a repair kit; for adjuster failure and housing damage, manufacturer documents generally call for caliper replacement.
What is a brake caliper, and how does a truck caliper differ from a car caliper?
A brake caliper is the part that carries the brake pads and presses them against the two faces of the rotating disc during braking. In passenger cars the force comes from a piston pushed by brake fluid. In air-braked heavy commercial vehicles, compressed air fills a brake chamber; the chamber's push rod pushes the operating lever inside the caliper, and the force is multiplied mechanically.
On axles with a parking brake, the same caliper also works as the parking brake through a spring brake chamber. In the common design, as with many car calipers, the housing slides on guide pins attached to the carrier; this is called a sliding or floating caliper. Depending on the design there are single-tappet and twin-tappet calipers.
How does an air disc brake caliper work?
An air disc brake caliper converts the straight push of the brake chamber into a much larger clamping force with the help of a lever and eccentric. Braking takes place in this sequence:
- 1. When the driver brakes, compressed air fills the brake chamber and the chamber's push rod pushes the operating lever.
- 2. The lever turns on its eccentric bearing and pushes the bridge (the intermediate part that distributes the force) toward the disc.
- 3. The threaded adjuster spindles in the bridge and the tappets on their ends press the inner pad against the disc.
- 4. In sliding calipers, the reaction force makes the housing slide the opposite way on the guide pins and pull the outer pad onto the other face of the disc.
- 5. When the air is released, the return springs (in the brake chamber and inside the caliper) bring the lever and bridge back to their starting position; a small running clearance remains between the pads and the disc.
In most designs the automatic adjuster is linked to the operating lever. When the lever travels further than is needed to close the running clearance, the mechanism turns the threaded spindles a little and advances the tappets, so the clearance stays roughly constant as the pads wear. In twin-tappet calipers both tappets are advanced at the same time.
UN Regulation No. 13 (ECE R13), which governs brake approval for trucks and buses in Europe and many other markets, requires wear adjustment on service brakes to be automatic, with some exceptions. It also requires that pad wear can be assessed without removing the wheel, or that a sensor warns the driver; this can be a wire loop in the pad or a continuous wear sensor in the caliper.
Main caliper parts and what they do
A caliper consists of a mechanism that transmits force and sealing parts that protect that mechanism from water and dirt; when the protective parts are damaged, the mechanism is eventually affected too.
| Part | Function | Typical sign of failure |
|---|---|---|
| Operating lever, shaft and bearings | Transmits the brake chamber thrust to the bridge through eccentric motion and multiplies it | Lever does not turn freely or does not return fully; rust at the chamber mounting port |
| Adjuster mechanism | Advances the tappets as the pads wear to maintain the running clearance | Clearance too large or too small; adjuster spindle does not turn |
| Bridge, threaded spindles and tappets | Distribute the force to the inner pad | Clearance difference between tappets, rust, brake not releasing fully |
| Tappet boots, cap, seals and plugs | Protect the mechanism from water, dust and dirt | Torn or melted boot; cap or plug dislodged |
| Guide pins, bushings and dust boots | Let the housing slide freely in the axial direction | Housing will not slide by hand; excessive play and rattling |
| Carrier | Holds the pads and transmits the braking torque to the axle | Wear or grooves on the seating surfaces; pad binding in its pocket |
| Pad and wear sensor | Provide the friction surface; the sensor reports the pad limit | Warning lamp; pad at its limit or worn unevenly |
Internal adjuster parts such as gears, springs, bearings and seals are listed under brake caliper mechanism parts, and guide pins, bushings and dust boots under caliper guide pin and boot kits.
How to tell if a brake caliper is failing
Caliper failure shows up as drag that continues after the brake is released, heat at a single wheel, pulling to one side under braking and uneven pad wear.
| Symptom | Likely cause | Check |
|---|---|---|
| Excessive heat at one wheel, burning smell; reddish dust or whitish residue on pad and disc | Dragging brake: seized pin, running clearance too small, lever not returning fully, leak in the parking brake circuit or damaged spring brake chamber | Whether the caliper slides by hand, and the running clearance; brake chamber and parking brake circuit |
| Heat at both wheels of the same axle or at many brakes | Brake balance problem, overloading, heavy brake use; a brake or relay valve that does not release air | The brake system as a whole before a single caliper |
| Vehicle pulls to one side under braking | Brake force difference between the two sides: one caliper not applying fully or the other dragging | Caliper sliding on both sides, pads and brake chambers; non-brake causes such as tire pressure and wheel alignment |
| Marked thickness difference between inner and outer pad | Worn guide pins or housing unable to slide | Pin play and axial caliper movement |
| Tapered (wedge-shaped) wear on a pad | Wear on the carrier seating surfaces, pin and bushing play | Carrier surfaces, pins and bushings |
| Pad wear warning | Pad at its limit, or sensor and wiring damage | Pad thickness; sensor, cable and connector |
If the heat is at a single wheel, the cause is most likely in that wheel's caliper, brake chamber or parking brake circuit; a wheel bearing fault can create similar heat. Measure temperature with a non-contact thermometer rather than by hand, and compare it with the other wheel on the same axle. To find leaks in the parking brake circuit, follow the steps in our guide on how to detect a leak in an air brake system.
For a quick preliminary check, once the brake has cooled, chock the vehicle on level ground, release the parking brake temporarily and push the caliper in and out by hand only, without tools. A small axial movement shows there is running clearance and the housing is sliding. If there is no movement at all, the pins, running clearance and adjuster must be checked in detail.
Why does a brake caliper stick?
The common root of a stuck caliper is usually water and dirt reaching the mechanism or the pins; heat and incorrect servicing speed the process up. The main causes are:
- Torn guide pin boots: Water getting in rusts the pins; the housing cannot slide and the pad drags on the disc.
- Damaged tappet boot, cap or seal: Water reaching the mechanism can lock the threaded spindles and the adjuster.
- Leaking brake chamber: Water can enter the caliper housing through the chamber; the brake applies with difficulty or does not release fully. The chamber is renewed at the same time so the problem does not return.
- Adjuster spindle turned with an electric or impact tool: The adjuster mechanism is damaged and can no longer set the clearance.
- Parking brake side problems: A leak in the circuit or a damaged spring brake chamber can keep the brake partly applied.
- Carrier and pad: The pad can bind in its pocket on worn or rusty seating surfaces.
Heat feeds this process: the heat of a dragging brake damages the boots and can even melt them, and water entering through the torn boot makes the sticking worse. At lubrication points such as the pins, use only the grease specified by the caliper manufacturer.
Repair kit or complete caliper?
For boot, pin and bushing problems a repair kit is generally enough; for adjuster or housing damage, manufacturer documents mostly call for caliper replacement. Problems caught before corrosion starts can be fixed with caliper repair kits.
Caliper replacement comes up when the adjuster does not work, when water has entered the mechanism and caused rust, when the tappets have lost synchronization or when the housing is cracked. Kits also exist for renewing the internal mechanism, but this job requires opening the caliper, a clean workshop environment, special tools and knowledge of adjustment and testing.
A pad change is the most convenient time to check the caliper. Look at the following:
- Sliding: With the pads removed, the caliper should slide freely by hand, with no excessive play against the carrier.
- Adjustment: The running clearance must be within the range given by the manufacturer. With the brake cold, the vehicle chocked and the pads fitted, back the adjuster spindle off as far as the manufacturer describes and apply the brake several times; the spindle should advance in small steps. If it does not, the mechanism is faulty.
- Sealing: Tappet and pin boots, caps and plugs must be intact; some manufacturers require the adjuster cap to be renewed every time it is removed.
- Disc: Thickness and cracks are judged against the manufacturer's acceptance criteria; a crack reaching the edge is generally not acceptable.
- Carrier and pads: Seating surfaces should be unworn, and the pads should go into and out of their pockets by hand. Pads are replaced together on both sides of an axle.
Safety: Before starting work, chock the vehicle on level ground and secure it. If a spring brake chamber is to be removed, the spring inside must first be locked mechanically with the release (caging) bolt specified by the manufacturer; once the spring is caged, the parking brake is out of action at that wheel and the vehicle is held only by the chocks. The spring is under very high force, and the chamber must not be opened by untrained people.
Do not remove hoses or chambers without first releasing the air pressure from the system. The disc and caliper can be very hot right after driving. Brake work must be done by trained personnel according to the vehicle's service documentation.
How to choose the right caliper and repair kit
The right caliper or repair kit is identified by its part number, not by its looks. Even on the same vehicle model, different calipers may be used depending on axle position, rim size, mounting arrangement, brake chamber and sensor equipment. Collect the following before choosing:
- Caliper label: The manufacturer part number and serial information on the housing.
- OEM number: The vehicle or axle manufacturer's part number; cross-referencing is done with this number.
- Chassis number (VIN) and model year: They pin down the vehicle's brake equipment and build period.
- Position and equipment: Front, rear or trailer axle; right or left side; whether there is a wear sensor and a parking brake chamber.
When choosing a repair kit, compare the contents list with the caliper code; pin kits and mechanism kits contain different parts. Complete calipers and caliper repair parts can be searched by OEM number. For how to read the number, see how to find the right spare part using an OEM number.
If the brake drags, one wheel gets noticeably hot or the vehicle pulls to one side when braking, have it checked at a brake workshop before using the vehicle again. If the wear warning is on, do not put the check off.