Answer :
Final answer:
To test the trailer spring/parking brakes only, one must pull out the red "tractor protection valve" button and apply the trailer handbrake. This is because this action will cut off air supply to the trailer and engage the trailer's spring brakes.
Explanation:
The correct procedure for testing trailer spring/parking brakes involves managing the air valves to ensure a safe stopping mechanism for the trailer. The air system used in trailer brake systems typically includes a red tractor protection valve and a yellow parking brake valve.
To test the trailer spring/parking brakes only, you need to pull out the red "tractor protection valve" button (also known as the trailer air supply valve) and apply the trailer handbrake.
This action cuts off air supply from the tractor to the trailer and engages the trailer's spring brakes. If the red valve button was pushed in instead, air would be supplied to the trailer and the spring brakes would release, which is not the intent of this test.
