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Old 05-31-2020, 10:13 PM   #1
Rainsmik
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Water on bathroom floor after draining black tank

I have a 2018 3811MS. Last 3 times I drained black tank, I found about a glass worth of water on the floor. It appears to originate on the floor of the cabinet located across from and directly to the rear of the commode. I drain the tank at the 2/3 mark and usually turn on the black tank wash when the flow from the tank slows down to a large trickle. I don't know how the water can get up through the floor, into the cabinet and then run into the bathroom floor. Doesn't seem possible, but it's happening. Any ideas?
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Old 05-31-2020, 10:24 PM   #2
Carl n Susan
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Welcome to one of the frequent Montana problem. There is an anti-siphon device, for the black tank flush system, under the sink cabinet. Open the cabinet doors and look for two white plastic lines. Those are the lines to the anti-siphon POS. The red and blue lines are cold and hot water lines to the faucet.

There are lots of solutions to your problem. Replace the valve with a better one or eliminate the valve altogether. Do a "Search" on "black tank backflow" or such and you should find a lot of information. Here are some samples:


https://www.montanaowners.com/forums...+tank+backflow
https://www.montanaowners.com/forums...+tank+backflow


I choose to cut the line, eliminate the valve and splice the line back together with Shark Bite fittings. You can easily find the water lines for the flusher behind the pass thru wall. I use an anti-siphon valve on the black tank flush hose connection in the convenience center.
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Old 05-31-2020, 10:25 PM   #3
PeteandJoan
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Originally Posted by Rainsmik View Post
I have a 2018 3811MS. Last 3 times I drained black tank, I found about a glass worth of water on the floor. It appears to originate on the floor of the cabinet located across from and directly to the rear of the commode. I drain the tank at the 2/3 mark and usually turn on the black tank wash when the flow from the tank slows down to a large trickle. I don't know how the water can get up through the floor, into the cabinet and then run into the bathroom floor. Doesn't seem possible, but it's happening. Any ideas?
I assume the cabinet you refer to is the one with a sink & faucet. Open the door and look up under and behind the sink, follow the water lines, there should be 4 lines, 1 for hot, 1 for cold, and the other 2 go up and are joined together with an anti-siphon valve. That line and valve is the water line used to flush your black tank. That valve is plastic and is very famous for leaking when you flush your black tank. Replace it with a good brass valve and you will have no more trouble. We all have had this problem.
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Old 05-31-2020, 10:26 PM   #4
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oops, Carl types faster than I do!!!
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Old 06-01-2020, 09:26 AM   #5
Rainsmik
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Thanks for the quick responses

Sorry for the delay in getting back to you. Posted and went to bed. The cabinet that I am referring to is not the one with the sink. It is located next to the hallway but within the bathroom. The sink is on the opposite (exterior) wall. Could the back-flow valve still be the problem? And, if not, any ideas?
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Old 06-01-2020, 09:34 AM   #6
PeteandJoan
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Originally Posted by Rainsmik View Post
Sorry for the delay in getting back to you. Posted and went to bed. The cabinet that I am referring to is not the one with the sink. It is located next to the hallway but within the bathroom. The sink is on the opposite (exterior) wall. Could the back-flow valve still be the problem? And, if not, any ideas?
Did you look under sink to see if it is there? If not some have been hidden in walls . If under sink, try your black tank flush and see if it leaks.
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Old 06-01-2020, 09:55 AM   #7
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Yes, they are not all under the sink. Some are located in walls (look for an access panel) and others are in other cabinets and sometimes the access panel is found in another cabinet.
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Old 06-01-2020, 02:25 PM   #8
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Look for a valve similar to this with two water lines to it.
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Old 06-01-2020, 04:31 PM   #9
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When you find it take it off and throw it away and splice the two pipe ends together.
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Old 06-02-2020, 05:18 AM   #10
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are you sure it is water and not bad aim? Asking for a friend
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Old 06-02-2020, 10:49 AM   #11
Tom N OH
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Get down on the floor, open that cabinet door & look on the left side of the inside of that cabinet. There will be a square cover panel with 4 screws holding it on. Take out the screws & take the cover panel off. The valve pictured above is inside there. That is the source of your leak. As others have said, it an anti-siphon valve for the black tank flush. Very common for them to leak. You could eliminate it altogether, or you can go to Home Depot & get a brass version which is much more reliable. Most of them are under the sink, but on the 3810-11-12-13MS, they put it in that wall since the sink is in the slideout. I found ours on our 3811 out of curiosity as to what was behind that cover.
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Old 06-06-2020, 10:39 PM   #12
Rainsmik
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That made me laugh!
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Old 06-07-2020, 03:48 PM   #13
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Quote:
Originally Posted by Rainsmik View Post
Sorry for the delay in getting back to you. Posted and went to bed. The cabinet that I am referring to is not the one with the sink. It is located next to the hallway but within the bathroom. The sink is on the opposite (exterior) wall. Could the back-flow valve still be the problem? And, if not, any ideas?
I also have an 2018 3811MS and I am flushing my black tank as I type this (on a water timer) I can hear the noise the backflow preventer makes coming from the towel/linen closet. Here are some pics. The hatch is in the bottom cubby hallway side.
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File Type: jpg IMG_4504.jpg (139.8 KB, 17 views)
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Old 06-07-2020, 03:57 PM   #14
Tom N OH
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Yep. Looks exactly like the one in our 3811MS.
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Old 06-08-2020, 10:34 PM   #15
Traveltimers2
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I also have water leak when I backflush black tank. The leak is on the pipe under the buffet in my Montana 3790Rd. Does anyone know why the leak, or what is that pipe there for. It comes up out of the floor 45 degree up to wall under toilet and connects with the shower vent pipe. Any ideas what the pipe actually does or is for?
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Old 06-09-2020, 02:58 AM   #16
Tom N OH
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Originally Posted by Traveltimers2 View Post
I also have water leak when I backflush black tank. The leak is on the pipe under the buffet in my Montana 3790Rd. Does anyone know why the leak, or what is that pipe there for. It comes up out of the floor 45 degree up to wall under toilet and connects with the shower vent pipe. Any ideas what the pipe actually does or is for?
Again, it is a back flow preventer valve, or check valve. It is designed to prevent contamination of the fresh water source by backflow from your black tank when you shut off the flow from the source. If it is leaking, it is not functioning correctly & needs to be addressed. Some remove the backflow preventer altogether, some replace it with a more robust & reliable brass one from Lowe’s, Home Depot, etc. I recommend the 2nd course of action. It’s a very simple replacement if you get the valve, an elbow & 2 Sharkbite type fittings.
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