Each air brake chamber with a spring brake is like two chambers in one: the usual service brake chamber and a spring brake chamber attached to the rear of the service brake chamber. To test for leaks, you need to fill both of these with air. To fill the spring brake chambers, release the parking brakes by pulling out the yellow button on the dashboard. To fill the service brake chambers, press and hold the brake pedal. Then listen for the hiss of escaping air and check the rate of air loss