The laws of physics are absolute. If it's not venting then there is something that is preventing it from doing so. If at all possible run a length of wire through the vent hole to pierce whatever is blocking the line. If that doesn't work and you have a compressor then remove the gas cap so you don't pressurize your gas tank and then use an air gun at the end of the airline to try blowing high pressure air through the vent line. Presuming that it used to vent then my guess is that a bug of some kind has built a home in your line over the winter and it's now blocking it.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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