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.