Minnesota CDL Air Brakes Test cheat sheet

Compressed air is the energy source: an engine-driven compressor charges the system, and stored air pressure - not a hydraulic fluid - applies the brakes.

The compressor is governor-controlled: it pumps until the system reaches cut-out pressure (about 120-125 psi) and resumes at cut-in pressure (about 85-100 psi).

A safety valve on the air tank (typically set near 150 psi) releases air if the governor fails and pressure climbs too high.

The air dryer removes moisture and oil before air enters the tanks; a saturated dryer lets water reach valves and can freeze or corrode them.

The foot valve (treadle valve) is the driver's control: pressing it meters air pressure to the brake chambers in proportion to pedal travel.

A brake chamber converts air pressure into mechanical force; a Type 30 chamber at 100 psi produces roughly 3,000 pounds of push rod force.

Slack adjusters transfer push rod stroke to the brake shoes; push rod travel beyond the adjustment limit means the brakes are out of adjustment.

Spring brakes use a large coil spring to apply the brakes; air pressure must be present to hold the spring compressed and the brakes released.

A dual system has two separate air circuits (primary and secondary); each can apply some braking if the other loses pressure.

In a standard dual system the primary circuit typically serves the rear brakes and the secondary serves the front brakes.

A loss of pressure in one circuit should not drain the other; if both circuits fall below about 60 psi the spring brakes apply automatically.

The low air pressure warning (buzzer and light) must activate before pressure drops below 60 psi.

The parking (spring) brake control is a yellow diamond-shaped push-pull knob; pulling it releases air and applies the spring brakes.

The trailer supply valve is a red octagon knob; pulling it closes the trailer supply line and applies the trailer spring brakes.

A tractor protection valve keeps the tractor's air from escaping through a broken or disconnected trailer line and applies the trailer brakes.

The front brake limiting valve ('slippery' setting) reduces front brake force; it is not a substitute for driving slower on slick roads.

Before driving, drain moisture from all air tanks; the supply tank is drained first, then the primary and secondary tanks.

With the engine off and the system fully charged, a static leak test: release the parking brake, hold the foot brake, and watch the gauge for one minute.

In the static leak test, pressure loss should not exceed 3 psi in one minute for a single vehicle or 4 psi for a combination.

Applied leak test: with the system charged and the engine off, press and hold the foot brake and check that loss does not exceed 4 psi in one minute.

Low pressure warning test: pump the foot brake to reduce pressure and confirm the warning sounds before the gauge reads 60 psi.

Spring brake test: with pressure below 60 psi, the spring brakes should be fully applied; start the engine and let pressure build, and they should release near 100 psi.

Check push rod stroke against the vehicle's adjustment limit; a stroke at or beyond the limit means the brakes need adjustment before driving.

Check the slack adjusters, hoses, drums, linings, and the air compressor belt; a loose or damaged belt can stop air from building.

Total stopping distance = perception distance + reaction distance + brake lag distance + effective braking distance.

At 55 mph a vehicle travels about 81 feet per second, so every second of delay covers a lot of ground.

Air brake lag adds roughly 0.4 second (about 32 feet at 55 mph) between pedal application and brake chamber pressure.

Perception and reaction time together are commonly estimated at about 1.75 seconds, adding roughly 142 feet at 55 mph before the brakes work.

Braking distance grows with the square of speed: doubling speed from 30 to 60 mph roughly quadruples the braking distance.

Brake fade from heat, wet brakes, and poor traction all lengthen stopping distance beyond the ideal dry-pavement figure.

Use engine braking and a lower gear on long downgrades; do not ride the brakes, which builds heat and causes fade.

Select a safe speed before a downgrade and brake firmly enough to feel the brakes slow the vehicle, then release to let them cool.

Brake fade is loss of braking caused by overheating; the pedal may feel normal while stopping power drops, so use gears and a safe speed to prevent it.

If brakes fade on a downgrade, downshift, use engine braking, and apply the brakes hard enough to slow the vehicle; do not continue at speed.

Wet brakes pull to one side or grab unevenly; apply light pressure while moving to heat and dry them, then test the brakes.

If the low air pressure warning sounds, move to a safe stop immediately; below about 60 psi the spring brakes will apply on their own.

If a brake chamber or air line fails, the dual system lets the other circuit brake; stop as soon as it is safe and do not continue the trip.

A rapid hiss of escaping air or a pressure gauge that keeps falling means a leak; stop and find the source before driving further.

If the spring brakes apply unexpectedly, keep steering and move off the roadway; the spring brakes cannot be released without air pressure.

Do not pump the brakes in an emergency with air brakes; a steady, firm application delivers full pressure to the chambers.

Always set the parking (spring) brakes when parked; the spring brakes apply automatically when air pressure drops, so they are the fail-safe parking brake.

On a downgrade, park with the front wheels turned toward the curb or the edge of the road, and chock the wheels if possible.

Never leave the vehicle with only the foot brake holding it; if air leaks away, the foot brake releases and the vehicle can roll.

When parking in cold weather, drain the air tanks to reduce moisture that can freeze in lines and valves.

Before backing under a trailer, check that the trailer spring brakes are set and the wheels are chocked.

After coupling, charge the trailer air system and test the trailer brakes and the tractor protection valve before driving.

If the vehicle must be towed or pushed with low air pressure, remember the spring brakes are applied; air must be supplied to release them.

Use a lower gear and the engine brake on long grades; the spring brakes are for parking and emergencies, not for controlling speed on a downgrade.