ln vehicles with three dash-mounted push-pull valve knobs, how are the valve knobs usually coloured?
- 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 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 yellow, the parking brake (system) knob is blue, and the trailer air supply knob is red.
Answer: C. The tractor park knob is blue, the parking brake (system) knob is yellow, and the trailer air supply knob is red.
On a three-knob dash, the tractor parking 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 the three dashboard valves on some tractors: the parking brake control valve is yellow, the trailer supply valve is red, and a separate round blue valve controls the tractor parking brakes by itself. Matching each knob to that description gives the blue-yellow-red arrangement. The same section also says the yellow parking-brake valve is normally tied to the red trailer supply valve, while the blue one lets you work the tractor's parking brakes independently.
Why the other answers are wrong
- A. The tractor park knob is blue, the parking brake (system) knob is red, and the trailer air supply knob is yellow. — This swaps the parking-brake and trailer-supply colors, but the handbook gives yellow to the parking-brake control and red to the trailer supply.
- B. The tractor park knob is red, the parking brake (system) knob is blue, and the trailer air supply knob is yellow. — This makes the tractor parking knob red and the parking brake knob blue, while the handbook puts blue on the tractor parking valve and yellow on the parking brake control.
- D. The tractor park knob is yellow, the parking brake (system) knob is blue, and the trailer air supply knob is red. — This makes the tractor parking knob yellow and the parking brake knob blue, but the handbook assigns yellow to the parking brake control and blue to the separate tractor parking valve.
Remember it
Picture yellow as the main parking brake, red as the trailer air line, and blue as the tractor-only park control.