PTO pump issue

Good morning all. I recently purchased an '88 L9000 dump. My issue is the dump won't lift. PTO will engage, and the air valve controlling the spool on the pump is clean and moving the full stroke. I noticed with the truck shut off, if I move the air lever in the cab to lift, I can hear air bubbling into the hydraulic oil reservoir. Could this be as simple as an o-ring on the spool valve, or am I looking at a new pump? Truck dumped fine when I bought it.
Thanks in advance.

PTO pump issue