A coupler for connecting the service port of a refrigeration fluid-containing system to an exit conduit has a stem threadably for axial movement within the coupling body between a first open position when fully advanced and a second closed position when fully retracted. A plunger axially extends from sealing means in the stem into a service port collar and unseats spring biased sealing means, permitting passage of refrigeration fluid, when the stem is in a substantially fully advanced position and the service port of a refrigeration fluid-containing system is positioned in a service port collar. When the stem is threadably retracted from a substantially fully advanced position, the plunger assumes a retracted position insufficient to unseat the spring biased sealing means and thereby prevents passage of refrigeration fluid.