how to fix my hydrstatic transaxle / ariens 1999 1340 sierra?

my lawnmower tractor 1999 Ariens 1340 sierra,drives slow,and going up a little grade stops completely. I have to pump the foot forward reverse pedal to get to move,eventually it will go at half pedal. I checked the drivbelt and it was good but still replaced it. what else do i do? i think i have water in the differential from washing it all the time, how do i change the fluid and what kind how much do i need or is it something else? it does have a push small cylnder lift like the ones that help to lift the hood on a car, that helps move the bracket to diff and footpedal control .thanks for your help. Samuel