Massachusetts CDL Permit Test - Air Brakes

In a vehicle with a single air brake system, after the engine is at operating rpms, the pressure in the air system should generally build from 50 to 90 psi within three minutes.

If the air pressure warning light or buzzer comes on while you are driving, you should immediately stop and safely park the vehicle. This warning signal indicates that there is a problem in the air brake system that needs to be fixed.

If a tractor is equipped with an Anti-Lock Braking System (ABS) but the trailer is not, the risk of jackknifing will be reduced and the driver should still be able to maintain steering control. If only the trailer is equipped with ABS, it is less likely that the trailer will swing out to one side.

All air tanks on air-braked vehicles must have one-way check valves located between the air compressor and the first reservoir. This prevents air from flowing out in the event of a leak.

In a dual air brake system, if one of the air systems gets very low on pressure, either the front or rear brakes will not be operating fully. You should bring your vehicle to a safe stop and have the air system repaired.

A parking brake should not be used if the vehicle's brakes are very hot from use on a steep downgrade or if the brakes are very wet and temperatures are freezing. Using the parking brake when brakes are hot could cause damage to the brakes, and using the brakes when they are wet in freezing weather may cause the brakes to ice up, rendering the vehicle immobile.