Strange happening with grey tank vent.

[shadows4]

Wife and I just got back from a 10 day,9 night, 3 Missouri State Park road trip. At the second park we had full hookups. I had my dump hoses hooked up but never opened any of the valves. I thought I would just dump all the tanks just before we left. Anyway the last morning I was taking my shower and heard a popping noise. Ended up coming from inside one of the bathroom walls. It was the vent pipe from the grey tank for the bathroom. I then went out and drained some of the grey water from that tank. The same thing happended whens the wife took her shower later. I checked the vent on the roof and the pipe had pushed the vent cover up a couple of inches. I did check it this morning before we left out third stop and the pipe had moved back down. Why is this happening and how do I fix it? Thanks, John

P.S. The second stop was Echo Bluff State Park. It is Missouri's newest state park so it had no trees but it is definately nice!!! They even have wild horses roaming through the park!!

[ScottG]

Those vent caps are notorious for being installed with the black pipe shoved up into them, effectively sealing the vent. There should be no way pressure can build up in that pipe.
Yours sound like they are the same. I would (and have on every RV I've owned) climb up on the roof, remove the white cap and trim some of the black pipe away so that it can breath.

BTW, the pop you heard was the screw that secures the white cap breaking free. There's nothing keeping the white cap on now when it's going down the road so you may want to address that while your up there. 😉

[agesilaus]

enblethen wrote:
Tank was bulging because it is full!

In my experience that means water runs back out into the shower drain.

[agesilaus]

Just an educated guess, but something has pressurized that vent line. And it should not be pressurized ever. So something is, at least, partially blocking the vent. I'd do a cleanup of the gray tank, dawn+calgon soaked overnight followed by flushes. Repeat until you get semi-clear water out the drain. It's possible that there is something solid in the vent itself, a rag or something that went down a drain. In that case you might have to push that into the tank via the roof vent using a long rod or short 'snake'.

Of course someone will probably point out something obvious that I missed.

[enblethen]

Tank was bulging because it is full!