S77 Hydraulics - can't lift load

When I hook up a 3", 8" throw cylinder on it and a small 8' tandem disk, the cylinder has enough to lift the wheels up, but not the disk. The cylinder cycles fine through its full range when operated alone and with no load.

What's the order of things to check as culprits for the lack of pressure under load? In searching archives, I've seen the relief valve, bad o-ring seals, to the worst option - worn pump gears or plates.

What's the simplest, easiest to check first and/or normal culprit here?

Aside from the o-rings and bearings which I can probably get most anywhere, how easy are the other parts like relief valves, pump internals to obtain?

I'm looking for things to try before going as far as yanking the pto shaft and pulling the unit out for teardown.

thanks everyone!

I'll check the relief valve seat for wear first. I've got the IT shop manual-not the full Oliver repair manual, but usually get through things okay with it and plenty of digital camera photos during disassembly (to remember how everything goes back together). :-)

Can't remember if the IT manual shows specs for the gears. If not, I'll just post again and ask for them.

Thanks again.