68 suburban hydro-trac

I have a 1968 hydro-trac, and don't think is working right. It has a higher pitched humming sound, and when I push the lever forward it seems to boge the engine down a lot, even with no load. Has anyone had the same problem and know how to fix it? I have tried to find a manual for the hydro unit so I can service it.

Has the fluid been checked or changed? There is a vent tube under the left fender with a dipstick in it or should be. You have to pull tire off, then remove I think a nut or something to pull out dipstick. I believe it uses auto tranny Dextron II or higher fluid.
Here is a link to a manual on the Sunstrand hyrdo rear. This is for Wheelhorse, but same rear was used on WH, Sears, and also a Massey 12 hydro, theory of funtion is all the same. You can also remove seat and plate to access adjustment for clutch linkages. Sorry, the owners manual for the hydros is like non existant.

got o engines/trans, scroll down to sundstrand 90 and download.
http://www.searstractormanuals.com/