How can you reduce tank pressure before monitoring the air compressor?

Reducing tank pressure before monitoring the air compressor is typically done by stepping in and off the brake. This action allows air to be released from the brake system, effectively lowering the pressure within the tank. When the brake pedal is pressed, air is used to apply the brakes, which helps to decrease the stored air pressure in the system. By utilizing this method, you can create a more accurate reading of the air compressor's performance and ensure that the system is functioning correctly.

The other choices do not effectively achieve the same outcome. For instance, releasing the clutch pedal primarily involves disengaging the engine from the drive wheels and doesn’t interact with the air brake system's pressure. Turning off the air supply would restrict the air from entering the system completely, which isn’t necessary nor practical for monitoring purposes. Engaging the emergency brake primarily activates a different braking mechanism and does not allow for a gradual release of pressure from the air tanks.