To test the tractor protection valve, pump the brakes with the engine off until the trailer air supply knob pops out, usually at _________.

Test tractor protection valve: Charge the trailer air brake system. (i.e., build up normal air pressure and push the 'air supply' knob in.) Shut off the engine. Step on and off the brake pedal several times to reduce the air pressure in the tanks.
The trailer air supply control (also called the tractor protection valve control) should pop out or go from 'normal' to 'emergency' position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).