Damn 30 year beat me to the post Jack the machine up and engage that side hydro, see if you get any movement at with no pressure on the tire. Before you spend the $600 on a Hydro unit, make sure you check the actuator and handle linkage. Make sure its properly engaging the Hydro unit. If this is your machine that your fixing, it can't hurt to take the hydro apart and attempt to find the problem.

1. Toro 74370 Service Manual
2. Toro Z4200 Timecutter Owner's Manual

Toro 74370 Service Manual

If you have no movement at all, more than likely its a pump issue. Going into the Hydro and rebuild/repairing it really isn't cost effective in a shop/labor situation like 30 year said. Well, 1 month later the hydro was replaced to the tune of 600.00 plus labor. Why don't they make these things service-able.

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Toro Z4200 Timecutter Owner's Manual

I noticed a year or so ago that it was slipping and still under warranty. I took it in and was told that the right side tire was going bald. I bought the new tires and it was good for a few more months. The hydro unit appears to be self contained with no access for checking the hydro fluid levels? Bad engineering or just something else that is disposeable? What can I do to prevent the same thing happening with the left side hydro?