What is the procedure to test the low pressure warning signal?

The correct procedure to test the low pressure warning signal involves turning on the electrical power and pumping the brake pedal to drop the pressure within the air system. This action simulates a scenario where the air pressure is decreasing, allowing the driver to observe the warning signal's functionality. The test ensures that the low pressure warning light or alarm activates at the appropriate pressure level, which is crucial for safety. If the warning system is functioning properly, it will alert the driver before the air pressure falls to a level that could lead to brake failure.

The other choices do not directly relate to the procedure for testing the low pressure warning signal. Simply increasing air pressure or releasing the parking brake could potentially lead to situations where the warning signal may not be accurately tested. Checking the gauges is more about monitoring air pressure rather than testing the warning signal itself, which is the focus of the correct procedure.