That was a heck of a mower in its day! Actually still not a bad rig.
I had a 1982 1600 Grasshopper that had the same thing going on with it, I ended up equalizing the levers the best I could and just used it that way for a couple more years until I upgraded to a new model. Its always good to change the hydro-fluid but in my case it didn't help it pull even. Good Luck.