Toro Timecutter Z420 left wheel

I have a Toro Timecutter Z420 for my small yard. This is the perfect size mower for my yard and it cuts great. I have recently noticed when i go to reverse with the left wheel it does not seem to have the same power as the right wheel. Sometimes if i pull the handle to reverse with that wheel it feels like it can't even move at all. I have not had to deal with any repairs on this yet so this would be my first attempt. I am pretty handy and feel like i can fix it I just do not know where to stand what what parts I might need to replace. Has anyone dealt with something like this on one of these mowers before? Any help would be appreciated. Thank you!

https://www.toro.com/en/parts/partdetails/?id=30088

see page 39 in operators manual , traction belts are worn loose or broken.

Thank you for the advice. I replaced the drive belt but I still seem to have the same problem. Any other suggestions? The wheel works but It definitely does not have the same power or speed as the right wheel.

Check that there is nothing preventing the levers on the tranny moving full distance.
Also check that the bypass valve is fully closed.
Purge the tranny by raising the bad wheel off the ground and with the engine at low idle, open the bypass valve and move the lap bar slowly 5 to 10 times from full forward to full reverse.
lose the bypass.
If this does not restore full ( or more ) power then check the oil level.
Make sure that the tranny is clean enough to lick before you open the filling port.
A very small amount of dust can destroy a hydro in no time flat which is why most are sealed.

If you do not have the Toro service manual for you mower, go to Toro & download it.
There are several manuals, download them all.
One will be for the hydraulics.