Farmall Cub

My 154 international cub's hydraulics bleeds down no matter what the position the selector valve is in. Thats the valve that selects either the 3pt hitch or the center rocker shaft. If i raise my mower with the up and down lever with the center rocker shaft, then change the selector valve to the 3pt hitch the mower slowly lowers. I have parked my tractor with the 3pt hitch raised and seen it lower too. It won't hold the attachments in the up position. Should i rebuild both valves? I can't see it being anything else, it must have a worn "O" ring.

I have been lucky so far my engine runs great and its been all little things wrong with my 154 cub mainly adjustements and maintenance.