How can you test that the spring brakes come on automatically?

To test that the spring brakes come on automatically, it is essential to simulate a condition where the vehicle's air pressure drops below the prescribed level. When you keep stepping on and off the brake, you are effectively using the brakes in a manner that depletes the air pressure in the system, simulating a situation where the pressure drops low enough for the spring brakes to activate.

In typical scenarios, the spring brakes are designed to engage when the pressure in the air brake system falls below a certain threshold, ensuring safety by preventing movement of the vehicle. This action of pressing and releasing the brake helps demonstrate whether the spring brakes respond as intended when air pressure is insufficient.

On the other hand, simply keeping the accelerator pressed or turning off the engine would not effectively test the spring brakes' functionality. Pulling the parking brake lever may engage the brakes deliberately, but it does not replicate the automatic engagement that occurs due to a decrease in air pressure. Therefore, stepping on and off the brake provides a practical way to evaluate the response of the spring brakes under stress conditions.