sealed hydrostatic transmission failing

I bought a 17 hp murray in 1999 with a foot operated hydrostatic transmission. My wife has put 400 hours on the mower, so far. This summer, my wife was complaining that it wouldn't climb hills. It finally got to the point that it would stop on flat ground. When I tried it, I found it did not pull at all. I cleaned the grass out of behind the clutch linkage with a small improvement. Then I put a new drive belt on (and deglazed the pulleys). More improvement, but it still would not pull a hill. I tried a 20 degree hill (that's 35% for those who prefer percent) and went up about 5 feet, then coasted back down. This suggested air in the hydro unit, or something slipping do to the pressure to force the speed change. I removed the transmission and tried to split the cases, but wasn't able to break the sealant loose. I did notice that the bellows indicated the hydro was overfull??? I also found a fill plug (a rubber second plug under the fan, as well as a larger plug in the gear portion of the unit). A large whoosh of air went thru the hole when I pulled the plug. I removed the four screws holding the bellows (t25 with a pin so a normal wrench doesn't work) by slotting the screws and useing a normal flat bladed screwdriver. Looking into the bellows chamber, I could see the oil level was down a bunch. I could also see that the pump was based on a round drum which slid up and down for forward and reverse. It must be a (awe memory!!) pump with the slideing plates. ie. it isn't a piston pump with a swashplate control. The hydraulic motor appears to be a similar unit without a variable displacement. As the unit has no recommendation for type of fluid (except it is a special synthetic), I tried to guess at the viscosity and thought it was in the 5-30 to 10-40 range. Since we have no reason to drive it when it is below 40F, I though a thicker oil would be OK. And I had some mobil 1 20-50 (ie V-twin motorcycle oil) which has a very high temperature rateing. I got about 1 quart out and put about 1 1/2 quarts back into the hydro unit. And made a special effort to get all the air out!!! The result is that my wife says it pulls better than new.
The point of this, -- I only need to pull the battery, -- pull the small rubber plug under the fan, -- and fill the hydro unit to get all the air out of the unit. ps. No leaks were evident, though the unit was covered in oily dirt.
JerryO
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