I think your system is air over hydraulic I don't know much about that system.
But if you have full air the parking brake should come on at about 60psi the remainder of the air in the system will apply the parking brakes. Trucks today have piggy back spring brakes on the air cans this is a mechanical parking brake and will hold the brakes even after all the air has drained from the system. If you have full air brakes without the spring brakes they can and should be added other wise don't ever forget to chock your tires because when all the air is gone so is the parking brake.
