Keep an eye on those Dorman plugs, I've used them on my ZRX and they started to dry rot in about a years time.
Lot of vacuum caps are meant to be used for testing purposes, not as a permanent install. The material might not do well exposed daily to sunlight etc. Where specifically on your ZRX did you use the Dorman plugs? Dorman caps are vinyl and vinyl might deteriorate even with the airbox location, dunno.
Anyways, should the PAIR spout on the Valk airbox be capped or plugged? Just thinking aloud .
Kawasaki used an air injection, or more like a vacuum that was prone to sucking engine oil, so it was an airbox cap also, I remember using them some place else, and same problem they would dry rot.
