VADEN ORIGINAL | Air Brake Systems | Commercial Vehicle Spare Parts

A trailer leveling valve holds ride height: it fills the air springs when load pushes the frame down and vents them as it rises. A lift axle valve decides whether a liftable axle carries load. Check the leveling valve and its linkage when height is wrong, and the lift axle valve circuit when an axle stays up or down. Last updated: November 19, 2026 Key takeaways Set ride height on level ground with over 100 psi in the system; Navistar's procedure finishes by adding air, never by venting. ZF-WABCO's mechanical lift axle valve lowers the axle at a factory-set 4 ± 0.2 bar of air spring pressure, protecting the other axles' load limits. A lift axle that drops when the ignition goes off is normal with a spring-returned valve. US rules require an air-suspended vehicle to sit level and leak no more than 3 psi in 5 minutes under 49 CFR 393.207. A leveling valve, also called a height control valve, is a lever-operated air valve on the frame, tied to the axle by a lin...

An air governor is the valve that switches the compressor between pumping and idling: at cut-out it sends reservoir air to the unloader, and at cut-in it vents that signal. Bendix lists D-2 governor bands from 90-105 to 120-150 psi. A bad governor shows up as no build-up, a venting safety valve or a dryer that never purges. Last updated: November 17, 2026 Key takeaways Settings follow the part number: Bendix catalogues D-2 bands from 90-105 to 120-150 psi, and FMVSS 121 sets only a 100 psi cut-in floor for trucks. Check the sensing line first: Bendix wants it at or near the top of the reservoir, and ZF-WABCO allows no air consumers on it. An adjustable D-2 moves both points together, about 4 psi per quarter turn; a wrong spread or a failed test means a new governor. Cut-out is the pressure at which the governor stops the compressor delivering air; cut-in is the lower pressure at which delivery resumes. The unloader is the part that obeys the signal. How does an air gov...

A relay valve applies the rear brakes from a tank when the pedal signals; a quick release valve vents chamber air at the axle when the pedal lets go. Soap both exhaust ports. On the service brakes, a leak with the pedal up comes from the relay valve, the only one holding tank air between stops. Last updated: November 12, 2026 Key takeaways A service quick release valve holds no air with the pedal up, so an exhaust leak then is the relay valve's. Bendix rejects a QRN quick release valve leaking more than a 1 inch (25 mm) bubble in 3 seconds at 100 psi. A blocked exhaust on either valve traps air and drags the brakes. FMVSS 121 timing limits certify new vehicles; they are not a workshop pass mark. A relay valve is a remote-controlled air valve: a small signal from the foot valve makes it send matching reservoir pressure to its chambers and later vent them through its own exhaust. A quick release valve is a simpler diaphragm valve with no reservoir feed that vents cham...

Bad fan clutch symptoms on a heavy truck depend on the drive. A failing viscous hub lets the engine run hot under load or keeps the fan roaring long after a cold start; a failing air-actuated on/off clutch hisses, slips or will not release. Check the fan clutch solenoid, air supply and wiring before replacing the drive. Last updated: November 10, 2026 Key takeaways Spring-engaged on/off clutches (Horton DM Advantage, Kysor) need at least 90 psi (6.2 bar) to release; an air leak leaves the fan on. Electronic viscous hubs (Horton VMaster Ultra, BorgWarner Visctronic) engage when unplugged, which splits hub faults from control faults. Cold-start fan roar can be normal: Navistar's VMaster guide gives the first release of the day up to 2 minutes at high idle. Replace on measurements: 0.050 in (1.27 mm) of rocking runout on Kysor hubs (Freightliner) and 1/4 in (6.35 mm) of DM friction facing (Horton). A fan drive speeds the cooling fan up when the radiator, charge air coo...

A tractor protection valve is the US part that keeps a tractor's air from following a broken or leaking trailer line: federal rules make it close automatically between 20 and 45 psi. European tractors split the same job between a four-circuit protection valve, which walls off a failed circuit, and the trailer control valve. Last updated: November 5, 2026 Key takeaways 49 CFR 393.43 makes the tractor protection valve act on its own once tractor air falls into the 20-45 psi band. ZF-WABCO's catalogue has no tractor protection valve; a four-circuit protection valve and the trailer control valve share its work. Quick checks: the red knob pops out at the maker's figure, and a braking bobtail tractor blows no air from the blue coupler. Air heard at the tractor end can start on the trailer: uncouple and listen again before replacing a tractor valve. What is a tractor protection valve, and what does it protect? It protects the tractor's own brake air. 49 CFR 3...