Answer :
Final answer:
The tractor protection valve will close automatically when the air pressure is too low, as a safety feature to preserve the tractor's braking capability.
Explanation:
The tractor protection valve is a critical component in a heavy vehicle's air brake system. When discussing the circumstances under which it would close automatically, it is essential to understand the functionality of such valves in the context of the air braking system. However, the detailed description you have provided about the input and output cylinders, red and blue valves, and oil flow seems to relate to a hydraulic system rather than an air brake system, and therefore does not apply to the tractor protection valve directly.
In the case of the tractor protection valve, it closes automatically when the air pressure is too low. This safety feature ensures that the tractor (truck) retains enough air pressure to continue to operate its brakes if the trailer were to become disconnected or if there is a significant drop in pressure.
