Hydro slipping on hills - Landlord

My mid 90's Landlord Hydrostatic - 18 hp BS VG just today starting slipping going up a hills. It gets up the hill but not as it should. I mowed with it yesterday and had all kinds of take-off. I checked the hydro release on the back of the tractor to see if it got pulled out and it is in the correct position.

reverse seems to work fine with no slipping.

Where do I start?

thanks