MO CDL Air Brakes flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-in pressure

The pressure at which the governor unloads the compressor and lets it pump again, commonly around 85 psi, restoring reservoir pressure after it has dropped.

Governor cut-out pressure

The pressure at which the governor stops the compressor from pumping, commonly between 100 and 125 psi depending on the vehicle's specification.

Air dryer function

A device that removes moisture and oil from compressed air before it reaches the reservoirs, preventing corrosion and ice in valves and lines.

Safety valve purpose

A pressure-relief valve on the supply reservoir that opens if pressure exceeds a set limit, protecting tanks and components from overpressure damage.

Treadle valve operation

The foot valve meters air to the brakes in proportion to pedal travel, so partial application produces partial braking rather than full on-off force.

Tractor protection valve

A valve that closes the air supply to the trailer when tractor pressure falls below about 45 psi, applying the trailer spring brakes.

Spring brake principle

Heavy-vehicle parking brakes are held applied by mechanical springs and released by air pressure, so losing air pressure causes them to apply automatically.

Dual-circuit protection

Two separate service brake circuits fed from the supply reservoir, so a failure in one circuit still leaves partial braking in the other.

Static leak test released

With the engine off and system charged, release the parking brake and watch pressure; a single vehicle should lose no more than about 2 psi per minute.

Applied leak test

With the engine off and a firm pedal held, watch for pressure loss and listen for leaks; allowable loss is higher than with brakes released but any steady leak is a defect.

Low-air warning check

With the engine off, fan the brakes and confirm the warning light and buzzer activate before pressure drops below about 60 psi.

Spring brake application test

Continue fanning off pressure and confirm the spring brakes apply automatically at roughly 20 to 45 psi, proving the emergency braking function works.

Build-up rate check

With the engine at governed speed, pressure should rise from about 85 to 100 psi within the specified time, typically under 45 seconds for a single vehicle.

Reservoir draining

Opening the drain valves on all reservoirs at the end of the day removes moisture and oil; drains must then close fully without leaking.

Slack adjuster check

With the brakes applied, measure pushrod travel; travel beyond the adjustment limit means the brakes are out of adjustment and the vehicle should not be driven.

Compressor belt inspection

Check the compressor drive belt for cracks, glazing, and correct tension, since a slipping or broken belt stops air production and can leave the vehicle without brakes.

Air-brake pedal lag

Air brakes respond with a delay between pedal movement and brake force, so the driver must apply the pedal earlier and more gradually than with hydraulic brakes.

Avoid fanning brakes

Pumping or fanning the pedal uses reservoir air with each application and can exhaust the supply, causing spring brakes to apply unexpectedly.

Downgrade braking technique

Select a low gear so engine braking and the retarder carry most of the load, and use short, firm brake applications instead of continuous light drag.

Brake fade

Loss of stopping force when overheated drums or discs reduce friction; a soft pedal or weakening brakes on a grade signals the need to stop and cool.

Following distance

Allow for air-brake lag plus the loaded stopping distance, keeping more space than a light vehicle would need at the same speed.

Pre-grade pressure check

Before a descent, confirm pressure is at governor cut-out and that spring brakes are fully released, since a partly released spring brake drags and overheats.

Parking with air brakes

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

Cold-weather moisture control

Drain moisture and keep the air dryer or alcohol evaporator working, because ice in lines and valves can delay or prevent brake release.

Low-air warning response

At about 60 psi the supply is nearly exhausted; stop as soon as it is safe because spring brakes will apply automatically at roughly 20 to 45 psi.

Automatic spring brake stop

When pressure bleeds off, spring brakes apply on their own; this is an emergency backup, not a planned stopping method on a grade.

Leak location by pedal

A rapid loss with the pedal released suggests a supply-side leak; with the pedal held, it suggests a delivery-side leak such as a ruptured chamber diaphragm.

Compressor failure response

If pressure will not build, stop and secure the vehicle, then treat it as out of service until the compressor or its drive is repaired.

Dragging brake hazard

A stuck or leaking valve can keep brakes applied; dragging brakes overheat and can cause fire or complete loss of braking.

Low-air driving response

Downshift, use engine braking and the retarder, and find a safe place to stop before spring brakes lock the wheels.

Broken air line effect

A ruptured line or failed chamber causes a sudden pressure drop and loss of that circuit, while a dual-circuit system retains partial braking on the other circuit.