When testing spring brakes, what activity should be performed before applying engine power?

The correct activity to perform before applying engine power when testing spring brakes is to remove tire chocks. Tire chocks are used to prevent vehicles from rolling when they are parked. Ensuring that tire chocks are removed is crucial for safety and proper testing of the spring brakes, as this allows the vehicle to respond to the brake system accurately without the risk of movement due to being confined by chocks.

Removing tire chocks is a standard safety protocol that prepares the vehicle for an actual test of the braking components. If the chocks are left in place, it might give a false sense of security regarding the brakes' effectiveness.

In contrast, draining the air tanks is typically done as part of routine maintenance rather than a test procedure, and checking brake fluid levels is not applicable to spring brakes, which operate on air pressure, not hydraulic fluid. Testing the emergency lights is also essential but unrelated to the initial preparations for checking the braking system. Thus, the focus on removing tire chocks lays the groundwork for a proper and safe brake system test.