I'm not sure how the drain works. There is a piece at the end of the tube about an inch long, it has a hex at the top, threds down the outside, and a small o-ring at the bottom. Is there a cap that screws on to this? Is the drain open without this cap? Thanks to any ideas.
Sounds like you are missing the cap. Should be the same type of cap as your low point drains. Put some water in the fresh water fill and see if it runs out. A 1/2" pvc cap will probably work.
dino, it's always helpful if you post year, make and model of RV you have, when asking for answers.
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Dan-Retired California Firefighter/EMT
Shawn-Musician/Entrepreneur/Wine Expert
and Zoe the Wonder Dog(R.I.P.)
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package
4pt Equal-i-zer WDH and 1828lbs of payload capacity