Don't hit the panic button yet. The old hydros are pretty sturdy units. If it was empty and you just put in a gallon, then it's still very low. If it were mine, I would hold off trying to get it to move and do some service on the unit.
Get a catch pan, and remove the rear plate. It's time to see what's going on inside, and clean out the gunk. Any number of things could be happening, such as a blockage. Clean out the globs, inspect and wipe a rag across the bottom of the case to check for metal shavings. Some photos may help determine where the leak is as well.
I have a feeling that a new filter (Napa #1410), fresh fluid and a good cleanout will have that thing running good again.
