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Old 08-31-2022, 08:42 AM   #1
JMGJR
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3855BR Black tank not draining completely

Anyone with 3855BR have issue draining the blank tank. Mine empties but it never seems to empty fully (based on how soon it is full again). Unless it is really full it never comes out with a big swoosh.
 
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Old 08-31-2022, 09:24 AM   #2
jsb5717
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Anyone with 3855BR have issue draining the blank tank. Mine empties but it never seems to empty fully (based on how soon it is full again). Unless it is really full it never comes out with a big swoosh.
How do you know when it's full? If you're using the little lights in your control panel then you aren't getting an accurate reading. Those lights are almost never accurate and most of us pay no attention to them. You will definitely know when the black tank is full. With our tank, and just the 2 of us, we can easily go 7-10 days without dumping it.

As for emptying, are you making sure you have plenty of water in the tank? A lot of water is one of the best ways to ensure that it clears out the tank well.
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Old 08-31-2022, 09:44 AM   #3
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Same issue on my Montana 3780RL, even with lots of water to increase the static head. Its a design flaw that does not provide adequate slope to fully drain in my opinion. Fortunately, the 2 gray tanks drain very well.
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Old 08-31-2022, 10:33 AM   #4
AZ Traveler
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You could have a poop pyramid built up. If so, put 20 gallons of water in to drive for a day before dumping. Doing this a few times can help break it down.

Never leave your black valve open and always start with a fair amount of water in the tank. Five gallons of water would be a minimum to put in the black tank after dumping and ten would be better.
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Old 08-31-2022, 10:52 AM   #5
Mikendebbie
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This simplified sketch sort of illustrates the problem with tanks draining faster or slower. Tank "A" is mounted in the drop frame section of the vehicle frame and it is at a lower elevation than Tank "B" which is mounted higher in the coach frame. Tank B is going to have more head pressure to empty quicker. That is the way is in my my RV. I have a black and a gray tank in the "B" location. They both drain great, whereas my black and gray tanks in the "A" location drain much slower. There are several youtube videos of Montana factory tours, and you can see the tanks mounted in the frame like this. In one of the videos the factory rep says we should raise up the front end of the RV to get better drainage out of the tanks in location A.
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Old 08-31-2022, 12:39 PM   #6
JMGJR
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I go by when the toilet starts burping when flushing to know it is full so I know it is. I follow up with 5 gallons of water. I only open black tank when dumping. I am guessing the diagram by @Mikendebbie is more likely the issue. Problem is we are seasonal with big family so I really do not want to clear up insides so slides can come in to raise the front end every few days
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Old 08-31-2022, 01:05 PM   #7
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We have a 2019 3130re. Your floor plan (bathroom) is similar to ours.
With our floor plan, we were told to get the forward tanks to drain and a good flush on the black tank, raise the front and tilt a little to the left.
Before winterizing, I lower the front and fill the black tank. Then raise the front as high as it will go and tilt it a little to the left so the sewer pipe drains too. Then pull the dump valve. Run the flush system and let it drain really good. To drain my galley tank I lower the front all the way.
Do the galley tank first or you will have to raise it again to empty the sewer pipe.
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Old 09-07-2022, 04:22 PM   #8
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Originally Posted by AZ Traveler View Post
You could have a poop pyramid built up. If so, put 20 gallons of water in to drive for a day before dumping. Doing this a few times can help break it down.

Never leave your black valve open and always start with a fair amount of water in the tank. Five gallons of water would be a minimum to put in the black tank after dumping and ten would be better.
If you use Unique Digest it tank treatment you should never have a poop pyramid. That stuff really keeps my black tank clear. You can also use some Dawn dish soap and about 5-10 gallons of water in between stops. That will also clean out your tank. I do that at the end of the season. That Unique doesn’t need a flush if just traveling between stops. Just add about 5 gallons back in, add more treatment, and you are done. Easy peezy.
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Old 09-07-2022, 07:05 PM   #9
Jritz
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I have a 2020, 3855br. From the very beginning my black tank didn’t empty completely. So what I do is, I raise the front up all the way and fill the tank with the flush out system and then let it go. It never really empties completely but I can get it to run clean. The gray tanks are fine. It is inconvenient but after putting 30,000 miles and staying in about 50 camp grounds I’m use to it and I’ve made it a part of my dump routine. I had it in to have it fixed our first trip out and they said there was nothing wrong with it.
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Old 09-08-2022, 08:08 AM   #10
Martin the Cat
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Anyone with 3855BR have issue draining the blank tank. Mine empties but it never seems to empty fully (based on how soon it is full again). Unless it is really full it never comes out with a big swoosh.
We just had the same issue. What we found was the steel bracket that was holding the one side of the black tank was installed with an extra 3/4" gap. Over time this allowed the tank to start to fall thru the side brackets. For us the rear side of the black tank had dropped 2-3 inches down. This meant that it would never empty right (because it physically couldn't).

We ended up using a bottle jack and a flat 2x4 to jack the tank back into place and then we moved the bracket so it better fit and held the tank up properly. You should check that out.

At the same time we also fixed the gray tank as it had the same issue of the bracket placement. Glad we caught them both before they fell off.
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