hydrostatic tranny questions

Hi guys, great forum... I am working on a mower for my son, the teacher... just replaced a fuel pump and fuel hoses to fix a start/die problem, now the mower doesn't seem to want to pull itself. Moves hardly at all in reverse and not a lot better in forward. It is a yardman model Z614H which he bought used a couple of years ago. He says it has worked fine until the last 1-2 times he used it when he noticed the loss of power, especially in reverse, but also in forward to some extent. For example, he said he hardly ever had to mow at full throttle before (i know, we already talked about that) but recently he has had to to make headway. According to the manual I found online, the tranny is a hydro-gear model 311-0510 or 618-0319, which are supposed to be the same unit. I checked the belts, they seem ok. I know this is supposed to be a sealed unit, but have read here and in other places that is possible to add/remove fluid. I'd like to try changing the fluid in this or perhaps just adding some first to see if it makes any difference. On the top of the unit there seems to be a plug which takes an allen wrench. Is this the fill plug? I tried to remove it with no luck but was afraid to try too hard, didn't want to strip it out, especially if it wasn't supposed to come out in the first place, lol. Anyone have any experience with this? I have found a few references to this in other threads, but wasn't sure they were talking about the same or a similar tranny to this one. I would really appreciate any thoughts, ideas or insights into this. Thanks in advance...