A service port cap has a body configured to be snapped onto a service port such as an automotive air conditioning service port. The cap has an axial slot allowing the body to spread as it is pressed onto the service port. A rubberized plug in the top of the cap bears and seals against the end of the service port. Should a fluid leak develop through the service port, the rubberized plug maintains a seal until the pressure is sufficient to overcome the seal created by the plug. At this point, the leaking fluid is directed out through the axial slot in the cap. Thus, the cap does not blow off and the flow of leaking fluid is controlled so that it does not spew. One embodiment includes a locking ring that slides to a locking position to lock the cap onto a service or other port to form a fixed seal.