Bathroom Leak From Black Tank Flush Line

[apcarch]

Hey all…am new to Montana ownership and hope to get some insight on a recent water leak. I have 2018 3921FB. When filling the fresh water tank, the wife yelled out’. “…there’s water pouring out in the bathroom…” out from underneath the bottom of the vertical cabinet right of the bathroom vanity (next to the bedroom wall). Photo 1. The source was coming from a white water line running vertically, in the wall chase, from the basement. Photo 2 looking up inside the wall chase (one can see the water droplets).
I have no idea where the line is going vertically and why it only leaks when filling the tank…???
When tracking the water line in the basement it runs horizontally, above the access door, toward the furnace location…from there not sure…
Any thoughts r welcome
PHOTO 1 is the bottom of the cabinet where the water was pooling. The line in question is in the wall to the right


PHOTO 2 leaking pipe in the wall chase, which shows 2 lines…I believe the one leaking is in the foreground

 

[Carl n Susan]

Are you sure this occurs when filling the fresh water tank? The fresh water tank is usually in the rear of the RV under the floor. It is filled by turning the shore water connection to "Fill".

Given the location of the leak in the RV and the presence of two white water lines, I believe this is the black tank flush line. There is an anti-back flow device under the bathroom counter in this system. They are known to have problems (Search is your friend). The key here is the two vertical white lines going up the wall. Any lines for the fresh water tank are horizontal, below the level of the floor and towards the rear of the coach.

 

[RMcNeal]

What Carl said. To fill the fresh water tank, you hook up your hose in the same place a city water, but there's a valve you turn to fresh tank fill. The black tank flush is a different hose inlet.
I removed the vacuum breaker/backflow device under the sink years ago and put a hose backflow preventer on the outside where the black tank flush connection is.
You might want to empty your black tank too if it's full.

 

[Mikendebbie]

apcarch - your pic #1 is taken in the front master bathroom, in the lower/rear closet cabinet. See my pic #1 below, there is a black access hatch with 4 screws. Remove this hatch to see the vacuum breaker for the front black tank tank rinse system. The white pipes are for the black tank rinse systems. My pic #2 shows the vacuum breaker. These OEM valves are notorious for leaking. Many folks replace the valve with a brass valve…just as many folks remove the vacuum breaker and tie the pipe together with elbows and small piece of pipe. I have not replaced mine and i am likely on borrowed time!

The vacuum breaker for the front bath black tank is generally behind the wet bay and can be accessed by removing the bulkhead walls and crawling back in among the rats nest of pipe and wire, and follow the white pipes. Each vacuum breaker will be installed at a “dimension” that is slightly higher than the toilet serving that black tank. When you look at your vacuum breaker in the front bath - notice its elevation compared to the front toilet…it is slightly higher than the toilet. The vacuum breaker for the half bath will be slightly higher than the nearby toilet.

My third pic is the wet bay. As stated above, to fill the fresh tank- hose should be hooked up to the city inlet and the black handle/switch pointed down to FILL. In the pic, my valve is pointed to NORMAL.

 

[apcarch]

Are you sure this occurs when filling the fresh water tank? The fresh water tank is usually in the rear of the RV under the floor. It is filled by turning the shore water connection to "Fill".

Given the location of the leak in the RV and the presence of two white water lines, I believe this is the black tank flush line. There is an anti-back flow device under the bathroom counter in this system. They are known to have problems (Search is your friend). The key here is the two vertical white lines going up the wall. Any lines for the fresh water tank are horizontal, below the level of the floor and towards the rear of the coach.

Ahhh…you are right on! I did use the lower right BW valve mistaken it for the fresh water fill… and good to know that and what the white piping serves. Lesson learned! No sign of the BF valve under sink so still looking( see my response to mikendebbie on the search)

 

[apcarch]

apcarch - your pic #1 is taken in the front master bathroom, in the lower/rear closet cabinet. See my pic #1 below, there is a black access hatch with 4 screws. Remove this hatch to see the vacuum breaker for the front black tank tank rinse system. The white pipes are for the black tank rinse systems. My pic #2 shows the vacuum breaker. These OEM valves are notorious for leaking. Many folks replace the valve with a brass valve…just as many folks remove the vacuum breaker and tie the pipe together with elbows and small piece of pipe. I have not replaced mine and i am likely on borrowed time!

The vacuum breaker for the front bath black tank is generally behind the wet bay and can be accessed by removing the bulkhead walls and crawling back in among the rats nest of pipe and wire, and follow the white pipes. Each vacuum breaker will be installed at a “dimension” that is slightly higher than the toilet serving that black tank. When you look at your vacuum breaker in the front bath - notice its elevation compared to the front toilet…it is slightly higher than the toilet. The vacuum breaker for the half bath will be slightly higher than the nearby toilet.

My third pic is the wet bay. As stated above, to fill the fresh tank- hose should be hooked up to the city inlet and the black handle/switch pointed down to FILL. In the pic, my valve is pointed to NORMAL.

Yeah that’s the correct location, however I have no access panel, black or otherwise ! Guess will have to do some experimenting exploratory openings around there that’s higher than the toilet.
Thanks for the heads in the other valve location for the half bath and clarifying the switch position for the fresh H2O fill method on this rig…much better than my previous one !
Will advise of my success with this illusive BF valve …!!!

 

[twindman]

Can you use a mirror and a flashlight to see if there is a backflow valve up there somewhere. I seem to remember something about people cutting a hole in the wall at the proper height to access the device. Is this wall on the back of the kitchen cabinets (I am not familiar with your model)? The older models owners actually cut a hole from the kitchen or somewhere and covered later with a plastic cover. Good luck.

 

[RMcNeal]

If you remove the panel from your pass through on the convenience center side, you can follow the white pex line and see where it goes. In my case, it had an elbow up under the sink to attach the valve, then an elbow down to the black tank. I cut out both and attached it straight to the tank. I then bought a backflow preventer that screwed into the inlet in the convenience center.

 

[Mikendebbie]

I plan to cut both vacuum breakers out and use a “hose end” backflow preventer on the hose I use for tank flush. You still need a backflow preventer somewhere for use with the tank flush systems…screw it onto your hose or like RMcNeal and put on the inlet. If I can’t get to the front vacuum breaker easily, I will cut the pex wherever I can reach it and connect the two pipes. Without the vacuum breakers, elevation no longer matters.