Your parking brake activates with no air pressure. When the air pressure bled off the brakes set. You have an air leak somewhere. Don't move the MH till the leak is fixed, you may have the parking brake engage as you go down the road if the air pressure leaks off.
Check the bleed valve on the bottom of the air tank, it may be clogged with rust if you don't bleed your tank regularly. Even with an automatic spit type valve, always bleed some air out of the tank at least once a week to make sure you don't have any water in the air tank. My guess, you have never bled the tank.
