Answer :
Final answer:
When testing the tractor protection valve, the red air supply control knob should be set to the emergency position. This valve is crucial for maintaining air pressure for the tractor's brakes if the trailer brakes fail.
Explanation:
When you test the tractor protection valve in a vehicle's air brake system, the red air supply control knob should go into the emergency position. The function of the tractor protection valve is to preserve air pressure in the tractor's braking system in the event of a trailer brake system failure or a breakaway. When testing the valve, the red air supply control knob acts as a quick release for the air supply, automatically setting the trailer brakes to prevent it from moving.
Detailed function: When the input cylinder is pushed downwards, the red valve closes and the blue valve opens, pushing oil past the blue valve, which in turn pushes the output cylinder upwards. Conversely, when the input cylinder is pulled upwards, the red valve opens allowing oil to be drawn in from the tank, and the blue valve closes to prevent backflow, keeping the output cylinder in place.
