Black Flush Line?

[masterdrago]

I'm thinking about removing the vacuum breaker under our sink. I sometimes smell stink from the black tank and want to eliminate possible leaks. The Thetford toilet has not been an issue (yet) - valve sealing and no drip from flush lever. So a couple of questions. Is the connection (pictured) have a check valve inside it? It looks like it does. Also what fittings do y'all use to connect the two tubes? I can either do it where the vacuum breaker is removed or under the floor where the tubes originate.