Jon,
I would guess that your leaking hydraulic pump wasn't something Chevrolet manufactured or installed on your truck. You would need to go to the manufacturer of the pump itself for parts.
Many times seals, o-rings, quad rings and other items can be obtained from a bearing supply house by merely ID'ing the part, or taking the part in for them to measure.
Scratch around and see if you can find the manufacturer and maybe a model or part number.
Is this pump for a dump bed?
Stuart
