ln vehicles with three dash-mounted push-pull valve knobs, how are the valve knobs usually coloured?
- The tractor park knob is red, the parking brake (system) knob is blue, and the trailer air supply knob is yellow.
- The tractor park knob is blue, the parking brake (system) knob is yellow, and the trailer air supply knob is red. ✓
- The tractor park knob is blue, the parking brake (system) knob is red, and the trailer air supply knob is yellow.
- The tractor park knob is yellow, the parking brake (system) knob is blue, and the trailer air supply knob is red.
Answer: B. The tractor park knob is blue, the parking brake (system) knob is yellow, and the trailer air supply knob is red.
The tractor park knob is blue, the parking brake (system) knob is yellow, and the trailer air supply knob is red.
Why this is the answer
The handbook describes each valve's knob separately. The spring brake control valve, which is the parking and emergency brake control, normally has a yellow, four-sided knob. Some trucks and tractors also have a separate optional tractor parking-brake control valve, and that one normally has a round blue knob. The trailer supply valve, which sends air to the trailer's brake system, normally has a red, eight-sided knob. Matching those three descriptions gives blue for the tractor park control, yellow for the parking brake system control, and red for the trailer air supply.
Why the other answers are wrong
- A. The tractor park knob is red, the parking brake (system) knob is blue, and the trailer air supply knob is yellow. — It swaps the colours around so the tractor park knob is red and the trailer air supply knob is yellow, which is the reverse of what the handbook says for those two valves.
- C. The tractor park knob is blue, the parking brake (system) knob is red, and the trailer air supply knob is yellow. — It gives the parking brake system knob red and the trailer air supply knob yellow, but the handbook puts the yellow knob on the spring brake control and the red knob on the trailer supply valve.
- D. The tractor park knob is yellow, the parking brake (system) knob is blue, and the trailer air supply knob is red. — It makes the tractor park knob yellow and the parking brake system knob blue, while the handbook assigns the yellow knob to the spring brake control and the blue knob to the optional tractor parking-brake control.
Remember it
Picture the shapes with the colours: the four-sided yellow knob is your main spring/parking brake control, the round blue one parks just the tractor, and the eight-sided red one feeds the trailer.