I also agree that it sounds like a coupler issue. Either incomplete engagement or as stated above, some internal coupler issues. Classic symptoms. I suppose you could remove the couplers and replace them with Hydraulic swivels.
Could you check pressures by hooking up a 2500 PSI gauge to the main supply hose, first one, then the other, hose, to confirm that there is pressure being delivered at the end of the hose? That would be simpler that tearing into valves.
Without using a gauge @ idle when I start tractor I have 4 to 5 foot of hydraulic fluid fountain so I can assume the pump is working..