You definitely don't want a restrictor valve in the line on a pfc hydraulic system.

The priority valve on those tractors worked well enough until your motor required either 18gpm or 1800 psi (give or take). If you went above either of those limits, then it would cut off all the oil to the remaining valves. If you have restriction in your line, then it's quite possible your priority valve pressure is above that pressure. That's why the advice to use the 3 and 4 valves helps. But, yes, fix the root cause as well.