Kitchen sink drain issues

[Daniellekristopher]

My kitchen sink drains amazing for the first 7 gallons. Then just stops. My only options to get the sink to drain at the point is to either (1) open the galley dump valve or (2) unscrew the vacuum break under the sink.
Note, when I open the vacuum breaker, a lot of vapor rushes out around the threads.
This suggests that there is simply too much back-pressure on the drain line. Once the tank is depressured, I can drain another 7 gallons, and the problem repeats itself.

I stuck a hose down the roof vent and let water flow down the vent pipe for an hour. I could not close the galley drain valve for more than a few minutes. Otherwise the galley tank level indication went to 100%. So the vent is apparently not clogged (as I original assumed).
I'm stumped, any advice ???

2018 Forest River Cardinal 3350RL
We dry-camp half the time. So leaving the tank drain open is not an solution when boondocking.

[Dwilcox]

It still sounds like a venting issue. The vent pipe could stick down into the tank and so water will run thru it but will allow it to back up fast. Can you stick a camera in there to check it?

[SlfMdeFam]

Originally Posted by Daniellekristopher

My kitchen sink drains amazing for the first 7 gallons. Then just stops. My only options to get the sink to drain at the point is to either (1) open the galley dump valve or (2) unscrew the vacuum break under the sink.
Note, when I open the vacuum breaker, a lot of vapor rushes out around the threads.
This suggests that there is simply too much back-pressure on the drain line. Once the tank is depressured, I can drain another 7 gallons, and the problem repeats itself.

I stuck a hose down the roof vent and let water flow down the vent pipe for an hour. I could not close the galley drain valve for more than a few minutes. Otherwise the galley tank level indication went to 100%. So the vent is apparently not clogged (as I original assumed).
I'm stumped, any advice ???

2018 Forest River Cardinal 3350RL
We dry-camp half the time. So leaving the tank drain open is not an solution when boondocking.

Plumber here... most likely what is happening is the drain pipe from the sink to the tank has a negative back grade on the pipe, not a gradual down hill slope towards the tank. Once you have enough water in the drain it creates air lock and the vacuum breaker(most likely not working properly either) not opening is causing this to happen. Hope that helps.

[Daniellekristopher]

I thought about the vent pipe dropping. But would assume the design of the vent pipe would be larger than the tank connection. Vent pipe being the female. And tank connection being the male. But I can’t validate that without seeing it.
So your suggestion is very possible, and the most logical.
Unfortunately I don’t see a means to get to the tank without removing 50 bolts on the under-blanket. Ugh.

[Daniellekristopher]

Regarding the air-lock theory, with that issue I would get a vacuum at the vacuum breaker. But instead, I am getting a pressure-buildup. Air is coming out when I remove the vacuum breaker.
But I will still replace vacuum breaker just in the one-off that’ll fix it. I will report back in a few day.

Also, sometimes air violently erupts up throw the undrained water in the sink. Especially on a hot day.

[Bama Rambler]

If air is 'bubbling' up in the sink, it's a vent issue. The fact that you get backflow of vapor out of the air admittance valve says that it's a vent issue also.

On most of those tanks, a hole is cut in the top of the tank and the vent pipe is just stuck in the hole and sealed up. there's nothing to keep it from being inserted too far except the technician's estimation.

[Daniellekristopher]

Thanks for the knowledge. I will get Into the tanks area over thanksgiving and reply with my findings and attach photos.
On the topic of the Cardinal 3350RL, I discovered while flushing water into the galley vent line that the Black Water tank and Galley grey water tank share a common vent. So it’s possible for black water vapors to flow into your kitchen if the P-trap losses water.
The bathroom grey water tank hasn’t it’s own vent.

[Thurman]

I will also state that you have a vent issue and the most likely cause in your case would be the vent stack (pipe) is into the tank too far.

[CaptnJohn]

Originally Posted by Thurman

I will also state that you have a vent issue and the most likely cause in your case would be the vent stack (pipe) is into the tank too far.

X 2 and quite common.

[SlfMdeFam]

I’m willing to bet after you replace the vacuum breaker it’ll drain fine