Christopher is totally correct. I've done several gauges and mostly thermal expansion valves (TXV). The link is basically how it is done.
The problem is mostly a broken or otherwise leaking capillary tube. Cut out the bad section out and splice or splice in a new section. Before final splice fill the bulb near full with ether. You may have to calibrate the gauge to make it more accurate if desired. Back in the day we used R-11 refrigerant which has a compatible temperature - pressure curve with nearly all gauges. Splices were silver soldered.