Illinois CDL Practice Exam.

To verify that spring brakes come on automatically in an air brake system, you would reduce tank pressure while the electrical system is on. This action simulates a loss of air pressure in the system, causing the spring brakes to engage as a safety measure. By reducing the tank pressure with the electrical system still on, you can ensure that the spring brakes are functioning as intended. Monitoring the brake pedal pressure (Option A) will not necessarily verify the automatic engagement of the spring brakes. Activating the Modulating Control Valve (Option B) is not a standard procedure for checking the spring brake system. Turning off the engine to test the springs (Option D) does not specifically check the automatic function of the spring brakes. Therefore, the correct method to verify that spring brakes come on automatically in an air brake system is to reduce tank pressure while the electrical system is on.