Neil,
There is a distinct possibility that there could be one or more seals in either the master cylinder or Airpak, that are not compatable with mineral oil, ie. for normal brake fluids. As the seals for the master cylinder are no different in design to a normal fluid type, also, I know of some applications where Airpaks are used, which work with brake fluid. ( seals used with wrong fluid can swell ).
Another common fault that will lock a brake on, is a sticking piston valve in the Airpak. My guess is your "REME guys" at the fort, might not have experience of Stalwarts, hence why they are reluctant :-D.