Yes cupcake is correct (Mario) ,,,,,
Try McMaster Carr, look up press gauge. $10 you will just have to figure out how you want to connect it. You don't have to keep it connected. Once you get the pressu where you want just disconnect it from the system. I'm not sure how your system is set up but like Mario said the relief valve needs to be in the system that the pressure can be diverted and not blocked. Usually the lower pressure is when the valve is all the way unscrewed. Most of these valves are a spring pushing down on a bb seat. (So to speak) the more the set screw is threaded in the more the spring compresses and the more pressure is needed to unseat it allowing oil to flow.
http://www.premacon.com/index.php/en...ystem-premacon
A simple secamatic from premacon describing the hydro system. Hopefully that may help.