Do the following test. Chock your wheels and build up your air. Shut engine off. Park brake NOT applied. Repeatedly apply the brakes via the foot treadle. Watch the air pressure gauge. At around 60 lbs. the low air light should come on and the low air buzzer should be buzzing. Keep applying the brakes and the Park brake should pop out. If you did this test and all worked then you are good to go. If the park brake did not apply you have a problem that needs to be fixed right away. If the buzzer did not come on then that too should be fixed.
Moisheh