If this is a new hydro, it may need to go through the air bleed procedure. Jack up the rear wheels, pull the bypass, start the mower and move the control lever through the forward and reverse several times, then release the bypass lever, and then run through forward and reverse several times, You should be good to go at that point. Most hydros are shipped with the oil already installed, but you have to bleed them. Make sure you removed the shipping plug at the tip, and installed the breather tube from the old tranny also.
