As you know, but neck is bashed open to fit a big mouth can. As far as the vent goes, it's just an open hose routed up and zip tied next to the cap. It never hisses when I take the cap off and doesn't dump on the track. Stinks the hell out of the trailer, but after seeing your asspain it seems like a small price to pay. With a vented cap, it seems to me I ought to be able to simply fit a vacuum switching valve in the vent hose and have it open whenever ignition power is on.
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