Weak lifting power on fast hitch

My International 330 has a fast hitch with very weak "up" hydraulic power. The down power is great and will lift up the entire tractor with the engine at a dead idle. But if I have a heavy implement on the hitch, I have to race the engine and wait a minute to pick it up off the ground. Once the implement is picked up, it will stay up for a long time when the tractor is parked (quite a few weeks I would say, although it will eventually lower to the ground).

So, my question is, what should I fix first? The fast hitch has a hydraulic circuit with both a check valve (365452R91) and a control valve (365981R92). Is it possible that the check valve has some weird cross leakage that weakens the up lift? Or is the control valve the problem? Like I said, the control valve works great on the down stroke, so is it possible that the control valve would have plenty of pressure in one direction, but not the other? I'm also curious to know if these valves can be rebuilt in place by just pulling the spools out and putting new o rings in. Thanks for your advice, I could use some ideas here. This tractor is very important on my small farm.

i would be leaning to a bad o ring in the control valve that is letting the oil bypass on the up cycle. i put new orings in the load check on my 240u and it stopped the implement from creeping down. get the o rings from case i-h. i got aftermarket ones and the would not seal. i think the i-h o rings are slightly different. i had a shop put new seals in my control valve, (the guy was a retired i-h mechanic) he said there are a couple special tools you need when you take the valves apart because some springs go flying out. in a pinch, try flipping the hydraulic lines coming out of the load check valve. it would give you fast lift, but less power on the downstroke.

I fixed it! It was a bad o ring in the check valve. The check valve has pipe plugs at either end, and behind one of them is a spool ("PISTON check valve actuator" in the parts manual) with an o ring. My o ring was mostly gone, and I got the exact size replacement at my local hardware store. This is a very easy, in field job to fix. I'm relieved because I was nervous about taking the control valve apart and could not afford to have this tractor out of commission while I wait to plant winter wheat here. Now the hitch lifts like it's on steroids! Thanks for your comments, btw.