A heavy truck's air system is a basic and important set of components that should be spec'd correctly and then maintained properly.
It includes a compressor, hoses and piping, valves, tanks and other parts. The system operates the brakes - meaning the truck can stop as well as go - plus suspensions on axles and seats, horn and other accessories.

Moisture, oil and dirt can disrupt operations and cripple the truck, or even cause it to crash. That's why the compressor has a filter at its outlet that removes most contaminants. What it can't catch is grabbed by a condenser/separator and then an air dryer. Dryers are virtually standard on modern trucks, but check specifications anyway to be sure the truck has one, as well as the condenser/ separator if air volume will be high.

What moisture and contaminants still get in the system can be let out with drain valves in air tanks. This should be part of a driver's pre-trip inspection, and be noted on the inspection check list. Or technicians can do it during inbound or outbound inspections. Either way, draining should be done every day.

There should also be a means to keep water from entering the system at gladhands. This is usually a bracket on the rear wall of the cab or sleeper that air-hose glad hands can be clamped to when the tractor is bobtailing. Trailers and dollies should have dummy gladhands to clamp over their air connectors when they are detached from a tractor or leading unit.

These are among suggestions included in the Recommended Practice 617A, "Contaminant Elimination Procedure for Tractor, Trailer or Dolly Air Brake Systems," from the Technology and Mainten­ance Council of the Ameri­can Trucking Assoc­iations.

RP-617A lists a procedure for forcing contaminants out of truck and tractor air-brake systems. This is done in 12 steps, from chocking the wheels and removing air lines from reservoirs to blowing out portions of the system and checking valve diaphragms for oil. Obviously, only qualified people should do this.

There's also a 10-step procedure for cleansing the air system in a trailer or dolly. Air in these vehicles may be especially dirty because contaminants can enter through the gladhands, which are well downstream of tractor air dryers. Both procedures require using compressed shop air that's clean and dry. Otherwise, all you'll do is replace old contaminants with new ones.

If you like step-by-step instructions, TMC's RP-619A, "Air System Inspection Procedure," lists 20 points that should be followed in testing for contaminants, valve operation and other functions of the air system in trailers and dollies. It describes a test unit consisting of a 50-cubic-inch tank with a gauge and drain cock, a shut-off valve and two glad-hand connectors. Following the procedure and correcting any problems found should result in vehicles with smooth-acting brakes, assuming valves and foundation brakes themselves are well balanced. (TMC has other RPs for that.)

For more information on TMC and its Recommended Practices, visit http://tmc.truckline.com.

From the December 2010 issue of Heavy Duty Trucking magazine.