Air brakes on trucks use compressed air from the compressor, dried and stored in tanks, to apply the wheel brakes as the driver presses the pedal. The pedal does not push the brakes; it only controls the foot brake valve, and pressure to the brake chambers rises with pedal travel. Parking and emergency brakes are applied by springs and released by air, so a pressure loss applies them automatically.
In short:
- The air path starts at the compressor and ends at the brake chamber: in between it is dried, split into circuits, stored in tanks and controlled by valves.
- The service brake works with at least two separate circuits; if one circuit leaks air, the other keeps braking.
- The spring parking brake applies by itself when system pressure falls below a certain value.
- A trailer is braked through a red supply line and a yellow control line (blue in North America); if the supply line breaks, the trailer brakes by itself.
How do air brakes work, and what is an air brake system?
An air brake system takes its braking force from compressed air stored in tanks, not from the driver's foot. It is used on trucks, tractors, buses and trailers. The diaphragm in the brake chamber turns that pressure into push rod force, and force is pressure multiplied by diaphragm area.
In a drum brake, this force is multiplied again by the leverage of the slack adjuster, which connects the push rod to the camshaft. This way, the force needed to stop a heavy vehicle is produced without great effort from the driver.
When the pedal is released, the air in the chamber is exhausted to atmosphere through the foot brake valve or through a relay valve near the chamber; the hiss you hear when the brakes release is this exhaust. Parking brakes work the other way round: a powerful spring in the spring brake chamber applies the brake, and air compresses the spring to hold the brake released.
This principle is the same on a truck, a tractor, a trailer and a bus; the differences are in the number of circuits, the valve layout and the electronics. Some light and medium-duty trucks have hydraulic or air-over-hydraulic brakes (a system controlled by air that applies the brakes hydraulically); this article describes the full air system.
Air brake system components: how does air get from the compressor to the wheel?
Compressed air passes through four stages before it reaches the brake chamber: generation and conditioning, distribution and storage, control, and application.
Air path: compressor → air dryer / air processing unit → circuit protection valve (four-circuit) → circuit tanks → foot brake valve and parking brake valve → relay valve / ABS modulator → brake chamber → drum or disc brake
1. Generation and conditioning
The air compressor is a piston pump, driven from the engine by gear or belt, that produces all the compressed air in the system; when choosing air compressors and compressor parts, the drive, cooling and lubrication type must match as well. The governor is the valve that switches the compressor between loading and unloading according to pressure: when the system reaches cut-out pressure (the upper limit at which charging stops) it unloads the compressor, and when pressure falls it switches back to loading.
If the build-up time gets longer or oil shows up in the lines, what to look at is covered in the article on when to replace the air compressor repair kit.
The air dryer holds most of the water and liquid contaminants from the compressor in a replaceable cartridge; dry air lowers the risk of line freeze-up in the cold. On many newer trucks the dryer, pressure regulation and circuit protection functions are combined in one housing, and this unit is called an air processing unit (APU). The cartridge's job and the replacement decision are the subject of the article on how the air dryer works and how it is maintained.
2. Distribution and storage
Dried air reaches the tanks through the circuit protection valve, which separates the circuits from each other. The service brake circuits have separate tanks; the parking brake, trailer supply and auxiliary consumers are fed from a separate tank or directly from the valve, depending on the vehicle. The tanks store enough air for a few brake applications when the engine stops or the compressor fails. Water that collects at the bottom is drained out through a drain cock or an automatic drain valve.
3. Control
The foot brake valve is a dual-circuit valve with two sections, each fed from its own tank, that sends pressure to the circuits in proportion to how far the pedal is pressed. The parking brake valve applies the parking brake by exhausting the air from the spring brake chambers.
On long vehicles, the distance between the foot brake valve and the rear chambers delays the response, so relay valves are used at the rear axles. A relay valve, using the small control signal from the foot brake valve, quickly feeds air from a nearby tank to the chambers and exhausts it quickly when the pedal is released. On ABS vehicles, the ABS modulator valve ahead of the chamber adjusts the pressure of a wheel that tends to lock, under electronic control.
4. Application: brake chamber and wheel brake
Brake chambers are pneumatic actuators that turn compressed air into push rod force, mostly through a diaphragm; on rear axles there is usually a spring brake (combination) chamber, a service section with a spring section added. They should not be confused with the hydraulic wheel cylinder in a car. On the common drum brake, the push rod turns the slack adjuster and the S-shaped brake camshaft (S-cam), and the cam presses the brake shoes against the drum.
On a disc brake, the chamber pushes the caliper lever; the caliper's structure and failures are covered in the article on how an air disc brake caliper works. Which axle has discs or drums, and diaphragm or spring chambers, depends on the vehicle and its equipment.
Why are brake circuits separate? Service, parking and auxiliary circuits
Brake circuits are separated so that a single leak does not disable all the brakes. The service brake works with at least two independent circuits; on most vehicles one feeds the front axle and the other the rear, but the circuit-to-axle assignment varies by vehicle. A separate circuit goes to the parking brake and trailer supply. In North America this layout is usually described as a dual air brake system with a primary and a secondary circuit; a four-circuit protection valve is not fitted on every vehicle.
Another circuit feeds auxiliary consumers such as the engine brake (exhaust brake) cylinder, the gear shift and clutch servos, the cab suspension and the air seat. On trucks with air suspension, the air springs use the same air source; how air ride suspension works is covered in a separate article.
The four-circuit protection valve (also called a circuit protection valve) isolates a circuit that is leaking air and protects the pressure in the others: pressure in the intact circuits falls only as far as the valve's closing pressure.
While driving, air in the spring brake chamber compresses the spring and holds the parking brake released. When the parking brake valve exhausts this air, the spring applies the brake. If system pressure drops, the spring starts to brake by itself, so when the low pressure warning comes on, stop the vehicle in a safe place.
On many European-type vehicles, the graduated parking brake valve can also be used as an emergency (secondary) brake if one service circuit fails.
Safety: The spring section of a spring brake chamber holds a powerful compressed spring. Do not take this section apart; releasing the brake by mechanically caging the spring is done only on the manufacturer's instructions and by trained personnel. A mechanically released chamber does not hold the parking brake, so chock the vehicle beforehand.
The static pressure drop test and circuit isolation steps used to find which circuit is leaking are covered in the article on detecting a leak in an air brake system.
How are trailer brakes controlled from the tractor?
On a tractor-trailer, the tractor connects to the trailer with two air lines, coupled with gladhands: the red supply line fills the trailer tank, and the yellow control line carries the brake signal (on North American vehicles the control line is usually blue). The trailer control valve on the tractor sends pressure into the yellow line according to the signal from the service brake circuits and the parking brake valve.
The trailer brake valve (trailer emergency relay valve) on the trailer uses this signal to send air from the trailer tank to the trailer chambers. Do not confuse the two valves: one is on the tractor, the other on the trailer.
If the supply line breaks or its pressure drops quickly, the trailer brakes by itself.
The tractor's protection function (the tractor protection valve in North American terms) stops the tractor from losing its air through the open line; the details of this behavior vary by valve type and vehicle equipment. On modern tractors and trailers, braking is supported by an electronic braking system (EBS), and the trailer's ABS/EBS unit is powered through a separate electrical connection. ABS and EBS faults that show up as a warning light are covered in why the ABS light comes on.
The load-sensing valve (ALB) is a valve that adjusts brake pressure to the axle load: on leaf-spring axles it senses the load through a mechanical lever, and on air suspension through the air spring pressure. This prevents the wheels from locking when the vehicle is empty and lets full brake pressure through to the chambers when it is loaded; on EBS vehicles this adjustment can be made electronically.
If the brakes of the tractor and trailer are not compatible with each other, the trailer pushes or pulls the tractor under braking and the linings wear unevenly.
Air brake problems: which part does each symptom point to?
With air brake faults, a symptom usually points to a circuit rather than a single part; the table below gives the first place to look and does not replace a proper diagnosis.
| Symptom | Likely cause | First check | What to do |
|---|---|---|---|
| Pressure builds slowly or does not reach the cut-out value | Compressor output low; leak in a line; dryer or purge valve stuck open | Build-up time; pressure drop with the truck stationary | Check the compressor and dryer |
| Pressure drops quickly when parked | Leak at a fitting, hose, valve or chamber diaphragm | Static pressure drop test | Locate the leak and replace the part concerned |
| Water or oil comes out of the tank drain | Dryer cartridge saturated; compressor passing oil | The drain and the date of the last cartridge change | Replace the cartridge or repair the compressor |
| Parking brake will not release | Insufficient pressure; parking brake valve, spring brake chamber or its line | Pressure gauge and valve outlet | Repair or replace the valve or chamber |
| Brakes are slow to release after the pedal is released, or one wheel runs hot | Relay valve, chamber, slack adjuster or caliper sticking | Wheel temperature (non-contact measurement) and valve exhaust | Repair the valve, chamber or caliper concerned |
| Trailer brakes weak, late or not holding | Control line, gladhand seal, trailer control valve or the valve on the trailer | Gladhands and line pressure | Replace the seal or valve |
| Wheels lock when empty, weak braking when loaded | ALB adjustment or linkage; EBS load signal | ALB lever and linkage | Adjust or replace |
When checking wheel temperature, do not touch the drum or disc with a bare hand; after driving these surfaces can be very hot.
What should you look at in maintenance and when choosing parts?
The life of an air brake system depends on the air entering it being clean and dry and on the connections being sealed. Routine maintenance looks at:
- Whether water or oil comes out of the tank drains
- Whether the dryer cartridge is replaced at the interval the manufacturer specifies
- Chafing, cracks or leaks on hoses, fittings and gladhand seals
- The condition of the chamber dust boots, push rods and slack adjusters
- Whether the warning lights and the pressure gauge work
On a long downgrade, relying on the service brake alone overheats the drum or disc and reduces braking. This is why on heavy vehicles auxiliary brakes such as the engine brake and a retarder are used to protect the service brake from this heat load.
Safety: Before working on the brake system, park the vehicle on level ground, chock the wheels and stop the engine; do not remove a line, fitting or valve until the air in the circuit concerned has been released. Compressed air is dangerous to eyes and skin. Brake work is a job for trained personnel.
If the low pressure warning will not go out, if pressure falls quickly when parked, if the parking brake will not release or if the trailer brakes are slow to apply, see a shop before taking the vehicle on the road.
When choosing parts, use the OEM or part number, and verify together with it the VIN, the model year, the valve's number of circuits and port layout, the fitting size and the vehicle's ABS/EBS equipment. Searching by OEM number in the MPT catalog and comparing the result with this information makes it easier to find the right part among air brake system parts. To see which part works where in systems other than the brakes, see the parts of a semi truck guide.