If memory serves, that fitting is designed for a vacuum source.  I have never used it since I do not have any vacuum instruments.  I understand that it is designed to allow the valve to operate slowly to prevent the cabin pressure rising too rapidly and bothering your ears.

BTW, if your checking your cabin pressure on the ground, the wiring from the gear switch to the regulator provides a ground when the gear is down which means the valve will be open.  During my ground check, I just used packing tape to secure a thin aluminum sheet over the regulator.

Good luck, Bill H.