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As you continue to fan the brakes down during the test, what should happen somewhere between about 20 and 45 psi?
- A.The engine should stall automatically
- B.The ABS lamp should turn green
- C.The governor should cut the compressor in at full speed
- D.The tractor protection valve and trailer supply valve should pop out (close), and the spring (parking) brakes should come on
Correct answer
D. The tractor protection valve and trailer supply valve should pop out (close), and the spring (parking) brakes should come on
Why
That pop-out is the system's last-resort protection: it seals the tractor's air and applies the spring brakes before pressure is completely gone. If the knobs do not pop in that range during the test, the protection is faulty.
Manual topic: Combination Vehicles - Air Brake Tests · From the Combination Vehicles practice bank (CV-205).