After you've connected your tractor to a trailer, you should test the trailer service brakes by applying
- the hand control valve. ✓
- the treadle (foot) valve.
- the parking brake control valve.
- the trailer air supply valve.
Answer: A. the hand control valve.
You should test the trailer service brakes by applying the hand control valve.
Why this is the answer
The handbook says the hand valve lets you apply only the trailer brakes, separately from the tractor. After you connect the tractor and trailer, that separate control is what lets you check that the trailer service brakes respond. The handbook also says the foot valve is for normal service braking, and it warns not to use the hand valve for parking or for normal or emergency braking. So use the hand valve for this specific trailer-brake check, not as your everyday brake.
Why the other answers are wrong
- B. the treadle (foot) valve. — The treadle or foot valve applies the tractor brakes as well, and the handbook presents it as the normal service brake, so it does not give you the trailer-only application used to test the trailer service brakes.
- C. the parking brake control valve. — The parking brake control valve is the yellow button that applies parking brakes on both tractor and trailer according to the handbook, so it is not the control for testing trailer service brakes.
- D. the trailer air supply valve. — The trailer air supply valve is the red button tied to supplying air and interlocked with the parking brake control, and the passages do not describe it as a way to apply the trailer service brakes for a test.
Remember it
Remember it as a trailer-only check: hand valve for the trailer brake test, foot valve for normal braking.