hydro transmission problem

[6jack]

I recently replaced the Sunstrand HT90 hydro transmission in my Bush Hog tractor with a new old stock unit. The old one took quite an effort to push without opening the bypass valve and would drift slowly on an incline if the engine was shut off. The new one can be pushed with little effort and will roll down hill easily. All the check and relief valves are free and clean. I like to stop the tractor on the ramps when loading to clean under deck and it drifts back down. Any ideas where my problem might be ?
The old parking brake was a pin in slot arrangement and was hard to release when on the ramps. I replaced it with a band brake. It will not keep the tractor from drifting backward. Unit seems to work well otherwise.
 

[Roger in Iowa]

Hydros will not hold a vehicle from rolling. There is always a little bleed off and once all is bled, look out!

Need to fix the parking brake!
 

[JT]

Roger is correct, a hydro will not hold on an incline for very long before the start bleeding off/bypassing and start moving.
 

[6jack]

Thanks for the information.