CDL Air Brakes Inspecting Air Brake Systems Summary

Wearing gloves, pull hard on each slack adjuster you can reach. If it moves more than about 1 inch where the push rod attaches, it probably needs adjustment. Brakes with too much slack can make the vehicle very hard to stop.

Wrong choices

• More than about 1/4 inch: A small amount of movement is normal. The manual uses about 1 inch as the limit, not 1/4 inch.
• More than about 3 inches: Waiting until there are 3 inches of movement is far too late. The brake would already be badly out of adjustment.

Check the build-up rate with the engine at operating rpm. In a dual air system, pressure should build from 85 to 100 psi within 45 seconds. (If the vehicle has air tanks larger than the minimum size, build-up can take longer and still be safe. Check the manufacturer's specifications.)

Wrong choices

• Within 3 minutes: 3 minutes is the typical figure for older single air systems building from 50 to 90 psi at idle, not for dual systems.
• Within 10 minutes: 10 minutes is much too slow. Pressure could drop too low while you drive.

With the engine off and electrical power on, step on and off the brake pedal to lower the tank pressure. The low air pressure warning (a buzzer, light, or wig wag) must come on before pressure drops below 60 psi in the tank (or in the tank with the lowest pressure in a dual system).

Wrong choices

• 20 psi: 20 psi is far too low. By then the spring brakes may already be coming on, and the warning would be useless.
• 100 psi: About 100 psi is where the governor tells the compressor to start pumping again (cut-in). It is not the warning level.

With the wheels chocked and the engine off, step on and off the brake pedal to lower the tank pressure. The parking brake knob should pop out when pressure falls to the manufacturer's setting, usually in the range of 20 to 45 psi. This makes the spring brakes come on.

Wrong choices

• Usually between 60 and 80 psi: 60 psi is the low-air warning level. The spring brakes come on later, at a lower pressure.
• Usually between 100 and 125 psi: 100 to 125 psi is the normal working range where the governor cuts the compressor in and out.

Charge the system fully, turn off the engine, release the parking brake, and time the pressure drop. For a single vehicle, the loss should be less than 2 psi in one minute (less than 3 psi for a combination vehicle).

Wrong choices

• Less than 3 psi in one minute: Less than 3 psi in one minute is the limit for a combination vehicle with brakes released, not a single vehicle.
• Less than 10 psi in one minute: Losing up to 10 psi a minute means a serious leak.

The governor controls when the air compressor pumps air into the tanks. The compressor should stop pumping (cut out) at about 125 psi and start again (cut in) at about 100 psi. Always check the manufacturer's specification.

Wrong choices

• About 100 psi: About 100 psi is the cut-in pressure, where the compressor starts pumping again.
• About 60 psi: About 60 psi is the level where the low air warning must already be on.

When air is used and tank pressure falls, the governor lets the compressor start pumping again (cut in) at about 100 psi. It stops pumping (cuts out) at about 125 psi. Check the manufacturer's specification.

Wrong choices

• About 125 psi: About 125 psi is the cut-out pressure, where the compressor stops pumping.
• About 45 psi: About 45 psi is near the top of the range where spring brakes come on. The compressor should cut in long before that.

With brakes released and the engine off, the air loss should be:

1. Single vehicle: less than 2 psi in one minute
2. Combination vehicle: less than 3 psi in one minute

A combination has more hoses and connections, so it is allowed a little more.

Wrong choices

• Less than 2 psi in one minute: Less than 2 psi in one minute is the limit for a single vehicle with brakes released.
• Less than 4 psi in one minute: 4 psi in one minute is the combination limit for the brakes-applied test, not the released test.

Apply the brakes with 90 psi or more using the brake pedal. After the first drop, a single vehicle should not lose more than 3 psi in one minute (a combination vehicle: not more than 4 psi). More than that means a leak to find and fix.

Wrong choices

• 2 psi in one minute: 2 psi in one minute is the single-vehicle limit with the brakes released, not applied.
• 4 psi in one minute: 4 psi in one minute is the limit for a combination vehicle with the brakes applied.

To check slack adjusters (the arms that connect the brake chamber push rod to the brake), park on level ground, chock the wheels so the vehicle cannot roll, and release the parking brakes. With the brakes released you can feel the true free play when you pull on each adjuster.

Wrong choices

• On level ground, wheels chocked, parking brakes applied: With the parking (spring) brakes applied, the push rods are already pushed out, so you cannot feel the free play.
• Engine running and the brake pedal held down: Holding the brake pedal down applies the brakes, which also hides the free play. The engine does not need to run for this check.