5.3 Inspecting Air Brake Systems

5.3 Inspecting Air Brake Systems

You should use the basic seven-step inspection procedure described in Section 2 to inspect you r vehicle. There are more things to inspect on a vehicle with air brakes than one without them. We discuss these things below, in the order that they fit into the seven-0step method.

During Step 2 Engine Compartment Checks

Check Air Compressor Drive Belt (if compressor is belt driven). If the air compressor is belt-driven, check the condition and tightness of the belt. The belt should be in good condition.

During Step 5 Walkaround Inspection

Check Manual Slack Adjusters on S-Cam Brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjusters. Use gloves and pull hard on each slack adjuster that you can get to. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment. Adjust it or have it adjusted. Vehicles with too much brake slack can be very hard to stop. Out-of adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters.

Check Brake Drums (or Discs), Linings, and Hoses. Brake drums (or discs) must not have cracks longer than one half the width of the friction area. Linings (friction material) must not be loose or soaked with oil or grease. They must not be dangerously thin. Mechanical parts must be in place, not broken or missing. Check the air hoses connected to the brake chambers to make sure they aren’t cut or worn due to rubbing.

Step 7. Final Air Brake Check

Do the following checks instead of the hydraulic brake check shown in Section Two “Step 7: Check Brake System.”

Test Air Leakage Rate (Static check). With a fully-charged air system (typically 120 psi), turn off the engine, chock the wheels, release (push in) the parking brake button (all vehicles) and trailer air supply button for combination vehicles and time the air pressure drop. After the initial pressure drop, the loss rate should be no more than two psi in one minute for single vehicles and no more than 3 psi in one minute for combination vehicles.

(L)-Leaks

Test Air Brake System for leaks. With parking brake, (all vehicles) and trailer air supply button (for combination vehicles) and trailer air supply button (for combination vehicles) released (pushed in), apply firm pressure to the service brake pedal. Watch the air supply gauge and listen for leaks. After the initial pressure drop, the loss rate for single vehicles should be no more than three psi in one minute and no more than 4 psi in one minute for combination vehicles. If the air loss rate exceeds these figures, have the air system repaired before operating.

(A)-Alarm

Test low Pressure Warning Alarm and/or Signal. Turn the key to the on position. Rapidly apply and release the service brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the tank.

If the warning alarm/signal doesn’t work, you could be losing air pressure without knowing it. This could cause the spring brakes to activate suddenly. Only limited braking can be done before the spring brakes come on.

(B)-Buttons(S)

Check that the Spring Brakes Come on Automatically. Continue to rapidly apply and release the service brake pedal to further reduce air tank pressure. The trailer air supply button (if it’s a combination vehicle) and parking brake button should pop out when the air pressure falls to the manufacturer’s specification (usually 20-40 psi). This causes the spring brakes to come on.

Check Rate of Air Pressure Buildup. When the engine is operating at 1800 RPM, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer’s specifications.)

If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring an emergency stop. Don’t drive until you get the problem fixed.

Check Air Compressor Governor Cut-in and Cut-out Pressures. Pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check manufacturer’s specifications.) Run the engine at a fast idle. Te air governor should cut-out the air compressor at about the manufacturer’s specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut-in at about the manufacturer’s specified cut-in pressure. The pressure should begin to rise.

If the air governor does not work as described above, it may need to be fixed. A governor that does not work properly may not keep enough air pressure for safe driving.

Test Parking Brake. Stop the vehicle, put the parking brake on, and gently pull against it in a low gear to test that the parking brake will hold.

Test Service Brakes. Wait for normal air pressure, release the parking brake, move the vehicle forward slowly (about 5 mph), and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel, or delayed stopping action.

This test may show you problems which you otherwise wouldn’t know about until you needed the brakes on the road.

The driver must locate and identify all air brake components, perform the LAB (leaks, alarm and button(s)) and correctly check the service (foot) brake operation to pass the pre-trip inspection.

Safety First!
Jayson Zamora