Here are a couple YouTube videos which are informative:
One popular driveline parking brake is Meritor DCM 1204 (I believe the 4 designates the shoe width in inches - there is also DCM 1203 and DCM 1205). Note that this is only the drum and shoe assembly and actuating arm but does NOT include any adjustment mechanism. There is no internal adjusting mechanism. The rest of the actuating and adjusting mechanism, whether rod, electrical, cable, hydraulic or air operated, is supplied by the vehicle manufacturer.
In your case being air operated, I would expect your PB to be fully applied or released in 1-2 seconds. In addition, a correctly designed, installed and adjusted drivelive PB should be able to hold a fully loaded vehicle on an xx% grade. I don't recall what the xx% is, but it is spelled out in an FMVSS and the manufacturer must certify that the vehicle meets it. I suspect is at least 10%.
Gene