To correctly install a pressure gauge, pressure switch or transmitter, it is advisable to use 2 suitably configured valves. Or better yet, an integral block and bleed valve.
In normal operation, the block valve will remain open and the bleed valve closed. To disassemble the instrument, or verify the zero, close the block valve and depressurize with the bleed valve.
To test the mounted instrument, use the purge outlet, entering a known signal through it with the lock closed.