Oh boy, I wish I could be more help Kirk, but don't want to steer you wrong. I went through something very similar with my plow tail wheel cylinder. Actually drove me almost to drinking!! Mine was kind doing the same thing, and ended up being the "new quick " connection I had bought. Wasn't letting oil pass back through. That's a double acting cylinder you have that has been changed to a single. Could very well be the issue with the connection. Could you get a coupling, just a pipe coupling and take out the quick connect for now and straight line it. If it does the same, at least you can eliminate that connection point. Reason I say that is I spent BIG money on rebuilding my cylinder only to find out that stupid 20 dollar "new" connection was my issue. May not be yours, but a good test to eliminate that possiblity. Good luck my friend, I'm right there with ya on all the learning curves too!!😁👍🤞

You can get a conversion kit to make them 540 PTO.

You can't block off hydraulic lines, its a two way system, it has to return to tank, you need another line from your spool valve going to other end of the ram. There is letters by the inputs and outputs on the dpool valve (hydraulic lever) p is pressure, t is return to tank

Could be the cylinder also did you do a presure test on tractor side

I bet is your cylinder but also I would check the tractor side of things