Sam is totally correct
Yes I'm sure there is wear in the hydro, everything wears, but it is normal to have to add a little throttle going up hill and it will coast a little faster going down hill.
That is just how that style of hydro works.
it is inherant to a axial piston pump/motor If you would like to learn more, go here:
http://www.cushmantrackster.com/pay-pdfs/sundstrand.pdf
it is an old service manual with some really good explainations/pictures of how it works.