I tried what herk did and it just leaks like crazy I tried to seal with tape no luck I tried with special pvc compound still leaked so went the route I went also the valves are protected from the elements. the fittings are a black plastic on mine and the caps have a sealing washer had some adapters made but was unable to find a suitable washer to work.
2001 Ford F-350 DRW 7.3
2011 25 RL Wildcat
former fiver 1976 Fourwinds had for 35 years