Looking for Help identifying a Parking Brake Air Cylinder

I have a 1979 Hough H80B Payloader. When I got the Payloader the brakes didn't work whatsoever and there were multiple big leaks in the air brake system and the air system's governor was turned way, way up. So I puzzled out retrofitted power clusters, hooked up shop air, fixed the major air leaks, bled the brakes, adjusted the governor pressure to 120 psi, and the air over hydraulic brakes now work great! However, on the compressed air side the system builds pressure and works and holds pressure with the parking brake on, but with the parking brake off, the system slowly bleeds air pressure off until the parking brake sets itself. When working the machine, when the parking brake sets itself, I must idle the machine with the parking brake set long enough to bring the air system back up to enough pressure to be able to release the air brake and get back to work. I've been able to ignore this problem for the most part but steadily the problem is getting worse and it now requires my attention. This is to say that it seems like the diaphragm in the air cylinder is leaking and I will confirm this once the snow and ice melts using shop air with the machine off. To me, the parking brake air cylinder looks like it was made by Haldex (who also made the brake fluid reservoir, compressor, and air drier on this loader), but I cannot find a part number anywhere on it to puzzle out an equivalent. Any ideas on how to "tag and identify" this animal? I ask because there are a bewildering number of air cylinder variants.