Heavy-duty air suspensions like Hendrickson's PRIMAAX EX bring air cushioned comfort to heavy construction vehicles.

Air Systems

The air suspension system consists of tanks, air lines, hardware, leveling valves and air bags. Replace the air line filter according to the suspension system manufacturer's instructions. If there is no filter, install one. Drain your air tanks daily.

Inspect air springs for cracks, gouges, distortion, bulges, and chafing. Check air lines and fittings for leaks. To check the pressure protection valve, drain air tanks with the engine off. The suspension should be out of the system until brakes are sufficiently recharged. They have priority for air use.

Clean or replace the leveling valve inlet screen regularly, and be sure the leveling valve is at factory settings.

Clean air springs with soapy water or with alcohol. Once clean, they can be thoroughly inspected. Do not use steam or pressure on air bags. Also avoid organic solvents, as they can damage the springs.


Hangers, brackets, components, fasteners, and bushings comprise suspension hardware. Check them regularly for damage. Before putting a new vehicle into service, and again before 3000 miles, retorque all fasteners. Suspensions have a break-in period when U-bolts stretch and conform to components they are holding. Retorquing prolongs suspension life. U-bolts require the right size and SAE grade (5 or 8) fasteners.

Many suspensions use Huckbolts with swedged collars. Remove only with special tools or by cutting the collar longitudinally. Never use a torch. The heat can weaken the fasteners, brackets, and chassis. The TMC document discusses factors that affect bolt tension.

Bushings allow a temporary misalignment of components when the suspension is under stress, then return components to proper alignment. To avoid problems, bushings should be at neutral stress (no up/down or forward/rearward loads) when aligning air bags.

Replace bushings at the first signs of damage. Roller bushings resist damage and move more easily than elastomer and metal bushings.


Include suspension maintenance as part of your preventive maintenance routine to reduce ride complaints, harshness, vibration, and increased tire costs. For copies of the TMC Recommended Practices, call 703-838-1763.

Paul Abelson is a former director of the Technology and Maintenance Council of the American Trucking Association and is on the Board of Truckwriters of North America. E-mail

Height Matters

Ride height control is critical to properly operating any truck with air suspension. According to TMC RP-634, Ride Height Concerns and Adjustment Procedures for Air Ride Suspensions:

Altering ride height to adjust fifth wheel height or for supposed ride improvement will actually deteriorate ride quality and can affect drive train angles. If drive shaft angles are too great, torsional vibration increases in severity to the point where it will shorten component life and create vibrations felt through the cab. Altering ride height could void transmission and drive axle warranties.