During the combination-vehicle brake check, how does a driver test that the tractor protection valve is working correctly?

Explanation

To test the tractor protection valve, the manual has the driver charge the trailer's air brake system, shut the engine off, then pump the brake pedal repeatedly to reduce tank pressure until the trailer air supply control pops out, or moves from normal to emergency, at the pressure range the manufacturer specifies, typically within 49 CFR 393.43's 20 to 45 psi activation window for a trailer's automatic emergency-brake feature. This test procedure is topic-scoped to 49 CFR 383.111(c)(2), with 393.43 cited as supporting context for the activation range rather than as the tested fact itself. Watching for the ABS lamp confuses an unrelated electronic fault indicator with the valve's own mechanical pop-out behavior. Testing by driving and braking hard is an unsafe way to discover a valve failure instead of the taught stationary pre-trip test. Leaving the valve in the normal position the entire time would never reveal the pop-out behavior being tested.

Source: 49 CFR 383.111(c)(2)