Mechanic Glossary

Air Brake Governor

A control valve that monitors air system pressure and commands the compressor to load or unload based on preset pressure limits.

An air brake governor regulates air system pressure. Because the compressor spins constantly, it would build dangerous pressures if left unchecked. The governor connects directly to the wet air tank. When pressure hits the cutout limit (usually 120 to 130 PSI), the governor directs air to the compressor's unloader valves, stopping it from pumping. When you use the brakes and pressure drops to the cut-in limit (typically 100 PSI), the governor exhausts this control air, letting the compressor pump again.

Governors are simple, reliable devices, but they can clog with moisture and oil sludge. If a governor sticks, the system might overpressurize and pop the safety relief valve, or fail to build pressure at all, leaving you with locked parking brakes.

Frequently Asked Questions

It is typically bolted directly to the side of the air compressor body, or mounted nearby on the engine bay firewall connected by thin nylon air lines.
Yes. You can remove the plastic cap on the governor and turn the adjustment screw inside. Turning it clockwise raises both the cut-in and cut-out pressures.
This is often caused by a failing governor or a leak in the governor control line. The governor is rapidly switching states, commanding the compressor and dryer valve to cycle.
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