HTR,
The pressure tests I would like you to do first, will help determine if your pump supply up to the selector spool is okay. It's not conclusive without a flow test.
Some farm supply businesses like TSC even stock hydraulic components now and so gauges are quite inexpensive to buy. I use the liquid filled gauges. Maybe $60 would buy 2 gauges and a 3' line.
The pressure regulator valve does control pressure to the shuttle assembly, and I believe oil flow is through this valve first, then the selector valve. If you remove the valve body containing this valve, keep in mind that the gasket(upper gasket) can be installed backwards, with unknown consequences.
Also depending on machine serial # you may have either a 'converter dump in neutral circuit'(pre # 8314666), or a 'converter charged in neutral circuit'( #8314666 up). This determines the part # of the 2 upper gaskets used. Not sure what impact this has on machine operation if the wrong gaskets are used, but they are very different. Did you replace all control valve gaskets?
Look again, on the small metal decal, on the right side of the big cast iron steering column. I believe there are 2 lines of info with 580 B CK on the top line and the machine serial # on the bottom line. Use some fine steel wool carefully, as these stamped #'s can get painted over. Phil