The exhaust break system, like nearly every other "OEM" type, (including class 8 trucks/tractors), the break circuit is a collapsible series. Meaning, any number of incorrect conditions will prevent it from engaging. Sitting stationary is one of them. Electronically controlled modern systems require zero fuel input. You'll have to duplicate the correct operating conditions to "test" it, such as actually driving up to a minimum speed, then decelerating.