Z425 reverse slow/weak

I just acquired a JD Z425. Very clean and well maintained.
The reason the owner got a new one is because the reverse is not working properly.
The left wheel will barely reverse at all and the right wheel is only slightly better. Both are very weak.

What I've done so far:
Added fluid (it was a little low)
Mowed 4 acres to purge any bubbles (I don't think there were any)
Checked linkages (all are tight with no slack)

Any known problems? Any suggestions? Is there a rebuild kit for the "transmission"?

BTW: This is my first post

You have the Hydro-Gear ZT2800 Hydrostatic Transmissions. They use the same pump and motor to go forwards and backward. If you are having problems I would first change oil and filter. It uses 20w 50 castrol motor oil. Any name brand 20w 50 oil will work. Just running the mower will not prge air out of the hydro. To purge air raise rear tires off the ground and support unit on jack stands. Pull the bypass lever. Get on mower and start it. Now run in forward full speed to count of 10 the do the same thing in reverse. Do this 10 times. Now put bypass valeves back to drive position and repete steps I explained explained earler.

Thank you very much for the reply BlazNT.

Before I got your reply I pulled the linkage off each side and tightened them 3 turns. This made a significant improvement.
I mowed the yard again and I have to say... The more I use this mower the more I like it.

I plan to pick up a filter this week (If I can get to the JD dealer). Is there a filter available at Autozone or Tractor Supply that will work?

When I change the filter and oil, I will jack the mower up and purge. I still don't think there is air in the system but, I don't mind making sure.

Thanks again.

You may not have air now but you will after oil change. Im glad an adjustment help you out.
On the hydros, I opt for factory filters only. As for oil, I use 20w 50 not John Deeres over priced stuff.

BlazNT said:

You may not have air now but you will after oil change. Im glad an adjustment help you out.
On the hydros, I opt for factory filters only. As for oil, I use 20w 50 not John Deeres over priced stuff.

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The local (rural) dealer didn't have the filter in stock. I was able to order one. I'll get it changed this weekend.
I definitely appreciate the feedback.