Air Brake Practice Test 2026 – All-in-One Resource to Master Your Certification!

A malfunctioning supply line shut-off valve can lead to an inability to apply the brakes, which is a critical safety concern in air brake systems. This valve is responsible for controlling the flow of compressed air from the compressor to the air tanks and subsequently to the brake system. If the shut-off valve fails, it can prevent the necessary air pressure from reaching the brakes. As a result, the driver may find that the braking system is either compromised or completely inoperative, which poses significant risks for the safety of the vehicle and its occupants.

In contrast, the other options do not correctly describe the outcome of a malfunctioning shut-off valve. Increased air pressure in the system is not a typical result, as the malfunction generally leads to a failure in pressure build-up. Reduced engine power could occur from other mechanical issues but is not directly tied to the function of the shut-off valve. Improved brake response would also be counterintuitive since a malfunctioning valve typically impairs braking efficiency rather than enhances it.