I think (I removed this stuff from my bus many years ago) the hard line going to the twist handle valve is the feed line. If you trace that hard line it will go back to an air manifold (a little hex shaped block with several lines connected to it) mounted on the rear wall of the engine bay. If you take the hard line off at that manifold and remove the adaptor that screws into the manifold itself, you can replace that adaptor with a 1/4 NPT plug, and some teflon tape or paste. You can then remove that whole setup from the engine bay and toss it (or save it, the same pieces are used for the fan belt tensioning system so you would have spares).