I have an 08 Exmark with 1700 hours on it. After it sits for a while, it will start right up but the right wheel will not turn. It has done this to me several times and does it every time now. It takes about 3 or 4 minutes of moving the levers back and for before the right wheel will move. Once I get it moving it is fine. Hydro take then looks milky(air bubbles). After mower sits the hydro oil clears up. Dealer tells me it is sucking air. Any ideas??