Other than the charge pump I've not worked on one either. What's it acting like?
A little history first. I have had this unit for about 8 years, got it non running from a local IH tractor dealer. A fresh plug, fresh oil, pulled the carb bowl, and drained the water out of the gas tank, put gas in instead and it has run fine for me since--until this year.
It has always always been just a wee bit slow on the left side, always had to pull the right lever back just a little as the right one has always been just a little (but not much) "faster" at full tilt. of course I don't run it full tilt while mowing, just to and from the garage..
This year I did not even use it until the mowing season was 3/4 over because of some tilling and such I did in my yard, it was easier to just push mow around all that. I went to use it in Sept. at a friend's farm, mowed the lawn with it up around the house, probably 3/4 acre.
Went to use it the 1st day, and burned up the deck belt that connects the 3 blade spindles so I ran it back into the barn and shut it off.
Went back 2 days later with new deck belt, put it on, and all of a sudden the left side had no power at all, I could only do donuts to the left as the right hydro still works. I left it a few hours and went to a tractor show and it worked again weird. So I mowed with it, and my wife mowed with it while I switched to a push mower, and it started getting slower and slower on the left side and finally the left one died. Again relegated to doing donuts. At this point I lifted the seat and the left hydro was HOT. Let it sit a few hours, and it kinda worked again til it heated up and slowly died again. The last time I went out there and tried to move it to get my snow blower out, it was stone cold and the left side was dead.
At one time among all of this the oil tank went dry, so I filled it, bled it, and after it was full, the left one still gets hot and dies off once heated up. The right side does not get hot at all. I could not tell where the fluid went, I didn't see any obvious leaks, as short a time as went by for it to go dry, something should have been soaked, but it was bone dry.
The 1st 7 years I had this machine, it cut ALOT of grass, a he11uva lot more than just my puny yard/ just not "this" season..
The way this works is that the engine drives a "T" gearbox, and each leg of the "T" is connected to a hydro via a lovejoy connector.
Each lever operates one hydro, one for each wheel. Each hydro has a chain, driving the wheel on that side of the machine.
Whereas a GT has a single hydro direct coupled to a transaxle, a ZTR has 2 hydros.
The older models of this unit, had an oil reservoir atop each hydro. this one has a common reservoir feeding oil to both hydros.