If the air pressure is still there it means that the internal piston seal is in tact. Also, if the gauge registered a pressure consistant with the normal pressure when the unit is filled (pressurized) with oil, compressing the air on the other side.
If it dumps the oil that fast you probably blew something outside the accusump. No oil spill? What kind of one way check valve are you using just before the T off for the accusump? Sounds like that one blew and the oil went straight out the oil pump instead of into the oil galleries
A single port plate? You have a hose feeding that accusump, which has a T in it, one side feed, one side accusump and the other side pressure to the engine? If something stuck you should have seen the air pressure stick instead of going to almsot 0 (or what you set it up to be empty .. 3-6psi??), is that what you mean with kept it's air charge? If that's what was happening indeed the piston is stuck and the oil is not coming out. I initially thought you meant by that, that the gauge was going down and stopping @ the preset air charge of several psi, in which case all the oil would have come out and the seal would be intact because the gauge shows pressure.
Why not pull it apart yourself? It's just a large aluminium machined tube with 2 end caps and a piston. The end caps are held in place with large C clips. Very easy to disassemble and inspect.
Internals on accusump are pretty basic, just aluminum seperator piston with some O-rings. I am going to dry-sump my 72 LT-1 dual porpose Vette (still ocassionally street drive it.) I know why my favorite high speed autocross series has "lost" their good site< because i have been gathering parts for over a year now to install dry sump set up