At the fill valves (high and low fittings) it has a center needle just like any shradder valve (valves to put air into tires). If you depress it it will let out any pressure that is in the lines (R-134a/oil), But it is illegal to release this into the atmosphere. . . Do what you will with that info.
My radiator is a Mishimoto, but I have built several with OEM radiators. The OEM will run right at the halfway point on the stock guage. Mine with the Mishimoto radiator sits just under the half way mark. I also run water wetter.