Cab Suspension and Related Components
Air cab suspension
Most highway tractors use a four-bag air cab suspension with shocks and a height control valve, or a spring-and-shock arrangement. The system's job is isolation: the cab should float without bottoming or leaning. Inspect air bags for chafing and leaks, the leveling valve and its link for bending, and the cab pivot bushings and mount bolts for wear and looseness. Cab height side to side should match OEM spec; a lean is a valve, link, or bag problem, not a spring rate problem.
Related components
Cab mounts, hood hinges, fender and step brackets, exhaust hangers, and engine mounts all contribute to cab noise and vibration. A vibration that appears only in a narrow speed band usually traces to tires, wheels, or the driveline; a rattle over bumps usually traces to a mount, bushing, or loose fastener. Check clearance between the cab, frame, air lines, and wiring before condemning the bags.
Rule of thumb
To separate a bag problem from a valve problem, disconnect the leveling valve link and stroke the arm by hand. If the cab rises and falls smoothly with no leak-down, the valve or its linkage is at fault; if one corner will not move, suspect the bag, line, or fitting on that corner.