49 CFR § 393.207 - Suspension systems.
(a) Axles. No axle positioning part shall be cracked, broken, loose or missing. All axles must be in proper alignment.
(b) Adjustable axles. Adjustable axle assemblies shall not have locking pins missing or disengaged.
(c) Leaf springs. No leaf spring shall be cracked, broken, or missing nor shifted out of position.
(d) Coil springs. No coil spring shall be cracked or broken.
(e) Torsion bar. No torsion bar or torsion bar suspension shall be cracked or broken.
(f) Air suspensions. The air pressure regulator valve shall not allow air into the suspension system until at least 55 psi is in the braking system. The vehicle shall be level (not tilting to the left or right). Air leakage shall not be greater than 3 psi in a 5-minute time period when the vehicle's air pressure gauge shows normal operating pressure.
(g) Air suspension exhaust controls. The air suspension exhaust controls must not have the capability to exhaust air from the suspension system of one axle of a two-axle air suspension trailer unless the controls are either located on the trailer, or the power unit and trailer combination are not capable of traveling at a speed greater than 10 miles per hour while the air is exhausted from the suspension system. This paragraph shall not be construed to prohibit -
(1) Devices that could exhaust air from both axle systems simultaneously; or
(2) Lift axles on multi-axle units.