Mystery water line

[alden3]

While draining my water system and adding antifreeze, I noticed one single line that did not get any of the pink stuff. It begins at my outside shower (both hot & cold faucets bleed pink) and ends by going up to what looks like a plastic vapor lock can in the bathroom vanity cabinet, and then goes back down under the floor somewhere. I seem to remember reading about something like this earlier. Does anyone have an answer to what it is?

[steves]

Its the vent for the black tank dump system. I don't think you should see any antifreeze in the line.

[NJ Hillbilly]

It self drains but I still blow it out to be safe.

John

[kdeiss]

Correct,that is the black tank flush just blow out no need to use pink stuff

[sreigle]

It's never a bad idea to blow it out but I never bothered with it - mostly out of ignorance, never even thought of it, and never had problems with it freezing.