have you tested for ANY hydraulic pressures? am I correct in thinking this unit has an internal [to the bell-housing] chain drive for the hydraulic pump? the "pressure switch on the spool block" should be for cutting out hydraulics (but not steering) when no one is on the seat, but if you are loosing steering, I would bet it is related to the steering [bottom of the hydraulic control valve, A.K.A. 'spool block'] pressure relief valve, if not the pump.