MO CDL Air Brakes cheat sheet

Foundation brakes are applied by mechanical spring force or air pressure; the compressor only supplies stored energy, so a loss of air pressure can leave the system unable to release or modulate brakes.

The compressor, governor, and reservoirs form the supply circuit: the governor cuts the compressor in near 85 psi and out near 100-125 psi depending on the vehicle's specification.

The governor controls compressor loading by sensing reservoir pressure; a failed governor can cause overpressure, rapid cycling, or failure to build air at all.

The alcohol evaporator or air dryer removes moisture and oil ahead of the reservoirs; a saturated desiccant cartridge lets water reach valves and tanks and freeze in cold weather.

A safety valve on the supply reservoir protects the system from overpressure and is typically set slightly above the governor cut-out pressure.

The foot valve (treadle valve) meters air to the delivery circuit in proportion to pedal travel; it is not an on-off switch, so partial pedal application gives partial braking.

The tractor protection valve and trailer supply valve control air to the trailer: the tractor protection valve closes automatically when supply pressure drops below about 45 psi.

Spring brakes are held off by air pressure and apply when pressure is released; they are the parking brake and the automatic emergency brake on most heavy vehicles.

Test for leaks with the engine off and the system fully charged: release the parking brake, hold a firm pedal application, and watch for pressure loss and audible leaks.

A static leak test with brakes released should show little or no drop; a typical pass criterion is no more than about 2 psi loss in one minute for a single vehicle.

With the brakes fully applied, allowable loss is higher than with brakes released; check the vehicle's specification, and treat any steady audible leak as a defect.

Perform the low-air warning check: with the engine off, fan the brakes and confirm the warning buzzer and light activate before pressure falls below about 60 psi.

Test the spring brake (emergency) function by continuing to fan off pressure; spring brakes should apply automatically when pressure drops to roughly 20-45 psi.

Check the build-up rate: with the engine at governed speed, pressure should rise from about 85 to 100 psi within a specified time, typically under 45 seconds for a single vehicle.

Drain all reservoirs at the end of the day to remove moisture and oil; check that drains close fully and that no air leaks from them afterward.

Inspect slack adjusters and pushrods: with the brakes applied, pushrod travel beyond the adjustment limit indicates the brakes are out of adjustment.

Air brakes respond with a lag between pedal movement and brake force, so apply the pedal earlier and more gradually than in a hydraulic-brake vehicle.

Use light, steady pressure and avoid fanning or pumping the pedal; each application consumes reservoir air and repeated fanning can exhaust the supply.

On long downgrades, select a gear low enough that engine braking and the retarder carry most of the load, and use the brakes in short, firm applications rather than continuous light drag.

Brake fade from overheated drums or discs reduces stopping force; if the pedal feels soft or braking weakens on a grade, stop and let the brakes cool.

Stopping distance increases with vehicle weight and speed; maintain a following distance that allows for the air-brake lag plus the loaded stopping distance.

Before descending a grade, check that pressure is at the governor cut-out level and that spring brakes are fully released, since a partially released spring brake drags and overheats.

When parking, set the spring (parking) brakes, place the transmission in a low gear or park, and chock wheels where required; air pressure alone does not hold a parked vehicle.

In cold weather, drain moisture and use the air dryer or alcohol evaporator as designed; ice in lines or valves can delay or prevent brake release.

A low-air warning at about 60 psi means the supply is nearly exhausted; stop as soon as it is safe because spring brakes will apply automatically at roughly 20-45 psi.

If the service brakes fail, the spring brakes will apply automatically as pressure bleeds off; do not rely on this as a planned stopping method on a grade.

A rapid pressure loss with the pedal released points to a supply-side leak; with the pedal applied, it points to a delivery-side leak such as a ruptured chamber diaphragm.

If the compressor fails, pressure will not build; stop, secure the vehicle, and treat the vehicle as out of service until repaired.

A stuck or leaking foot valve can cause brakes to drag or fail to release; dragging brakes overheat and can lead to fire or complete brake loss.

If the low-air warning sounds while driving, downshift, use engine braking and retarder, and find a safe place to stop before spring brakes lock the wheels.

A broken air line or failed chamber causes a sudden pressure drop and loss of braking on that circuit; dual-circuit systems retain partial braking on the unaffected circuit.

After any emergency stop caused by air loss, do not move the vehicle until pressure is restored and a full leak and function check has been completed.