Air brake system - Basics of heavy highway truck air brake

All heavy highway trucks and trailers are equipped with air brake system to slow, stop, or park the truck. The air brake system consists two components, service brakes and spring brakes. Service brakes are controlled with the brake pedal for the purpose of slowing or stopping the truck-trailer. Spring brakes or emergency brakes are controlled with bush/pull red and yellow buttons located on the truck's dashboard. The purpose of the spring brakes is to park or for in case of emergency stop. Spring brakes automatically engage if air pressure drops bellow 45 pounds per square inc (PSI). Compressed air stored in the air tanks is used to apply the brakes.

How do air brakes work?

When brakes are apply to a vehicle equipped with air brake system. The applications creates friction between the brake linings on the brake shoes and brake drums. This converts moving energy into heat by friction. The amount of heat required to stop the vehicle depends it's weight and speed. If you apply more pressure, it will create more friction which, brings the rig to stop quicker but also it means creating more heat. Application of brakes for extended period will also cause brake fade.

What is brake fade?

Brake fade occurs when brakes applied heavily and continuously for extended period, normally by going down a mountain and is caused by brake overheat. Over heated brakes will not stop or slow the vehicle effectively. The best way to go down a mountain is to apply and release brakes momentarily. Releasing brakes will give brakes time to cool down before you apply them again.

Weight and speed affects

Weight and speed both affect the distance and pressure required to put the vehicle into complete stop. The more the truck weighs, the more air pressure is needed to produce friction and heat to stop the truck. Speed also affects distance and pressure required to stop the vehicle.

Air brake components

Air brakes are operated by number of important components such as the air compressure, governor, air dryer, alcohol evaporator, air reservoir, protection and control valves, gauges, brake champers, slack adjusters, brake drums, glad hands, and warning devices.

Air Brake Component Functions

Air Compressor - a machine that pumps the air into a smaller space to produce compressed air powered by the truck engine and could be mounted to the engine or by brackets and fasteners.

Governor - A controller that limits the Air Compressor to produce amount of air required. It controls the compressor cut-in (on) and cut-out (off) to make sure the compressor produces enough air but not too much air. Governor is normally located on the compressor.

Air Dryer - Removes moisture and vaporized oil from the compressed air before it enters the air supply tank. Air dryer is normally located between the air compressor and the supply tank.

Alcohol Evaporator - Prevents any moisture in the air brake system to freeze. Frozen moisture could cause brake failture.

Air Reservoir system - Stores compressed air before it's released to the brakes. The air in the Reservoir is supply by the Air Compressor.

Safety Valve - protects the air tank to over pressure or release access of air in case the governor fails.

Warning Devices - low air pressure warns the driver when the air drops bellow 60 psi. A light and buzzer goes off to warn the driver in the case of law air pressure.

Air Pressure Gauges - pressure gauge located on the dashboard indicates amount of air pressure in the air tank. Some trucks are equipped with two air pressure gauges, one for each tank. Also some trucks have application pressure gauge indicating amount of air pressure applied to the brakes.

Brake Champers - holds or consists Diaphragm, Air Inlet, Push Rod, Return Springs, and Clamp.
Air enters the Brake Champers through the Air Inlet and pushes against Diaphragm which also pushes the Push Rod. The Push Rod is connected to the Slack Adjuster. Return Spring returns the Diaphragm to it's original position.

Slack Adjuster - a lever arm attached to the Push Rod.

Glad Hands - coupling devices at the end of the air lines where tractor connects with the trailer. Glad hands are color coded, service line is normally blue and supply line is red. Service line supplies air from the brake pedal to the trailer service brakes and supply line supplies air to the trailer air tank and controls trailer brake.

How To Check The Air Brake System Functionality

Physical components of the brake system should be checked while performing the daily pre-trip inspections. Besides the physical inspections of the brake system for damages, cracks, leaks, loose, and foreign objects, one should also check the air pressure build up time, Governor functionality, low air pressure warning, air loss rate, Safety Valve functionality, and brake functionality.

Secure the truck and trailer to prevent movement while performing the air brake test.
Check the air pressure build up time by reducing the air pressure bellow 80 psi. Air pressure build up time from 85 to 100 psi should be more than 2 minutes when the engine is running 600-900RPM
Observe the Governor cut-in and cut-out time. Normally functioning Governor should cut-out an air pressure range of 100 to 135 psi and cut-in above 80 psi.
Check the low air pressure warning by pumping the brakes while the ignition is on. Low air warning buzzer should come on when pressure drops but not bellow 55 psi. If low air warning comes on bellow 55 psi or not at all, then it's not functioning.
To check the air loss rate, securing the vehicle with chocks, build the pressure to cut-out position, release the parking brakes, shut off the engine, and hold the brake pedal for 1 minute while observing the presure gauges for air loss. Truck with trailer should not loose more than 4 psi of air in one minute or 3 psi for tractor only. Tractor with 2 or more trailers should not loose more than 6 psi of air.
Check the tractor protection vale by disconnecting the service line (blue) to the trailer and pressing the brake pedal and listining to any air leaks. There should not be any air leaks in normal operation.
Finally check the tractor and trailer spring brake functionality by applying one at time and trying to move slightly.