Originally Posted by sailingdog
Before doing any of that, it might be worth checking the existing vent line to see if it is clogged. Wasps and other bugs love to build in the vent lines. Also, if you've ever over filled the holding tank, it could have less desireable materials clogging it.
I have had this problem a lot. My tank is small, and the vent is right over the tapered end of the tank. Even if I didn't overfill that tank a spirited sail seems to be enough to clog the tube. I have made it a routine to reverse flush the vent line every time I pump out. This seems to be working so far... Use some old battery cable or similar stiff wire to clear any obstructions. Yummy...