CDL Air Brakes cheat sheet

The engine-driven compressor builds air pressure; the governor cuts the compressor out at about 125 psi and back in at about 100 psi.

A safety valve on the supply tank releases at about 150 psi and protects the system if the governor fails to cut out.

Air storage tanks hold compressed air and have manual drain valves at the bottom for removing water and oil; tanks are drained daily.

The alcohol evaporator adds alcohol to the air stream so moisture will not freeze in lines, valves, or the compressor.

A brake chamber converts air pressure into mechanical force that extends the pushrod toward the foundation brake.

The slack adjuster is the lever between the pushrod and the brake shoe; it multiplies force and takes up lining wear.

An S-cam (or wedge) rotates as the pushrod extends and forces the brake shoes outward against the drum.

An air dryer removes moisture and oil vapor before the air reaches the tanks; its filter and desiccant require periodic service.

The foundation brake is the drum or disc, shoes or pads, and hardware that creates the friction which actually slows the vehicle.

A dual air brake system has two independent circuits: the primary usually operates the rear brakes and the secondary the front brakes.

Each circuit has its own reservoir, supply lines, and check valves, so a failure in one circuit still leaves braking in the other.

Check valves allow air to flow in one direction only, preventing backflow and air loss through a failed circuit or the compressor.

The tractor protection valve closes automatically when the trailer lines separate or the trailer is uncoupled, keeping air in the tractor.

The red trailer air supply knob charges the trailer lines; pulling the knob out closes the tractor protection valve and applies the trailer spring brakes.

Spring brakes are held in the released position by air pressure; when pressure is lost, the springs apply the brakes automatically.

ABS is a separate system added to the air brakes; when ABS fails, normal service brakes still operate but wheels can lock.

A low air pressure warning and the automatic spring brakes are the two backups a dual system provides against total air loss.

Static leak check: with a fully charged system and the engine off, loss should not exceed 2 psi per minute for a single vehicle or 3 psi per minute for a combination.

Applied leak check: with the pedal held down, leakage should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

Build-up check: at fast idle with a charged system, pressure should rise from 85 psi to 100 psi in 45 seconds or less.

Low pressure warning check: the light and buzzer must activate before system pressure drops below 60 psi.

Spring brake check: with pressure below 60 psi, the parking brake knob should pop out and the spring brakes should apply fully.

Slack adjuster check: with the parking brake released, pull each adjuster by hand; movement of more than about one inch means it is out of adjustment.

Pushrod travel must be measured and compared with the limit for the specific chamber size; too long a stroke means the brakes are out of adjustment.

Drain each air tank daily and confirm only air and slight moisture come out; oil, sludge, or heavy water indicates a problem.

Inspect lines, hoses, glad hands, and fittings for cracks, chafing, and leaks, and confirm the pedal feels firm and does not sink to the floor.

Choose a safe speed and shift into a low gear before starting down a long or steep downgrade; stay in that gear all the way down.

Snub braking on a downgrade: apply the brakes firmly to reduce speed about 5 to 10 mph, then release and repeat rather than riding the brakes.

Brake fade results from heat built up by continuous or repeated brake use; the pedal must be pushed harder for the same stopping force.

Controlled braking: apply the brakes as hard as possible without locking the wheels, keep the steering wheel straight, and hold until stopped.

Stab braking (vehicles without ABS): apply the brakes fully, release as soon as the wheels lock, and reapply; never stab with ABS.

With ABS, apply steady firm pressure and do not pump the pedal; ABS allows steering while braking hard but does not always shorten stopping distance.

Stopping distance increases with speed, weight, and downgrades; wet or slippery surfaces reduce available traction further.

Engine braking through a lower gear supplements the brakes and reduces heat on long grades; it does not replace the service brakes entirely.

The low air pressure warning light and buzzer must come on before pressure falls below 60 psi; stop, find the leak, and do not continue driving.

Spring brakes begin to apply automatically when pressure falls into the 20 to 45 psi range and stop the vehicle as pressure continues to drop.

If service brakes fail, downshift to use engine braking, look for an escape ramp, and use the parking brake gradually rather than all at once.

On a tractor-trailer, a major air loss causes the tractor protection valve to close, preserving tractor braking so the unit can be stopped.

If the trailer breaks away, the emergency line loses pressure and the trailer spring brakes apply automatically.

An illuminated ABS lamp means ABS is not working; the service brakes still function, but the wheels may lock under hard braking.

Some vehicles have a modulating control valve on the dash that allows the spring brakes to be applied gradually for emergency use.

Escape ramps are gravel or sand beds built to stop a vehicle with failed brakes on a downgrade; keep the vehicle straight and enter at speed.

Always set the parking brake before leaving the vehicle; pulling the knob releases air and lets the spring brakes apply.

Never depend on air pressure alone to hold a parked vehicle; a slow leak will eventually let the spring brakes apply or the vehicle roll.

On a grade, set the parking brake and turn the front wheels toward the curb when facing downhill; use wheel chocks on heavy or steep situations.

Shift into a lower gear before the downgrade begins; never shift down partway down the grade or the vehicle may gain too much speed.

After driving through deep water, test the brakes at low speed; wet linings reduce friction and can cause a delay or pull to one side.

Dry wet brakes by driving slowly and applying light, steady pressure until braking returns to normal.

On slippery surfaces, release the brakes if the wheels lock; use gentle, light braking and engine braking instead of hard applications.

In freezing weather, drain the air tanks and use the alcohol evaporator to prevent moisture from freezing in valves and lines.