Sundstrand 15 U type tranny leak...

I bought a non-running John Deere 300 sometime ago and just finished rebuilding the engine. The engine runs fine now, so I hooked up the driveshaft, started it up, and immediately saw tranny fluid is leaking pretty heavily out of the the upper fitting of the suction tube (next to the filter), and I'm looking into why it could be leaking so bad. If anyone's got a guess, please pass it on. FWIW, the check valve adjuster is backed all the way out, but the tractor doesn't move when the lever is moved forward or reverse.

The manual indicates this is a 7/8-14 "O-ring" type of fitting - does this indicate there should be a separate O-ring on the fitting? The fitting's plenty tight, but a missing O-ring could certainly cause a leak.

If I'm reading the circuit diagram right, should be part of the suction flow to the charge pump. If the tractor doesn't move when the shifter is moved, and the suction tube leaks, could the charge pump be creating back pressure?

I haven't yet removed the fender deck to look at the check valves. They were rebuilt, but I suppose the plungers could be stuck in the down position, which would explain why the tractor doesn't move. Would this also explain the leak? Does running the engine/tranny with the check valve plungers depressed create an over-pressure situation which would cause a leak?

If any of you Sundstand experts have any ideas about where to start looking, let me know...