When was the lasted time you changed or checked the drive belt? A worn belt and/or worn idler tensioner pulley are the most common causes for this problem.
park on the top of the hill and release the brake
IF it stays there the hydro is good & the belts/pulleys are bad
If it races a way like mulga Bill's bicycle then the tranny is gone & needs a new motor - pump unit .
And of course partial wear between these two extremes