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Old 10-21-2018, 01:59 PM   #1
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Check valve for black water flush

I have a 2015 legacy and the black tank flush is not working. Iím sure itís the check valve but I have not been able to locate it. My best guess after tracking the lines is behind the toilet but I have not been able to remove that panel, appears to be screwed from the back. That leaves removing the metal tv box for the outside tv to find it. Anyone had to have this check valve replaced and if so do you know where it is? Thanks.
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Old 10-21-2018, 02:52 PM   #2
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Black tank flush doesn't have a check valve. They use what is called a anti-siphon valve, backflow preventer or vacuum breaker. Usually under the bathroom sink as it must be installed higher than the toilet bowl. Some may be installed in a wall.

In the first pic below the brass one has replaced the original, the black pipe next to it is the vent for the gray water tank. The black one in the second pic is most commonly used in RV's. You can see the bottom of the sink in the upper right of both pics.
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Old 10-21-2018, 05:14 PM   #3
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Thanks, now I know what Iím looking for. Mine is not under the sink. The waterline is running into the wall behind the toilet, that must be where it is located. Looks like I need to pull the outside tv compartment to gain access to it.
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Old 10-21-2018, 06:32 PM   #4
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Thanks, now I know what Iím looking for. Mine is not under the sink. The waterline is running into the wall behind the toilet, that must be where it is located. Looks like I need to pull the outside tv compartment to gain access to it.
The black tank cleanout will not be associated with the toilet water lines or in any other way with the toilet. It is a stand alone system that has tubing from the outside direct to the black tank. There will be a vacuum breaker/check valve in series with that tubing. Does your unit happen to have a washer/dryer cabinet adjacent to the bathroom and if so might check in there for the valve. Of course, it might be where you mentioned. The valve may not look like this one but the install should be similar.
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Old 10-21-2018, 07:09 PM   #5
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And when you do find it ,make sure you blow out the black tank flush line when winterizing, or you'll be replacing it again in the spring.
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Old 10-21-2018, 08:02 PM   #6
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And when you do find it ,make sure you blow out the black tank flush line when winterizing, or you'll be replacing it again in the spring.

I live in Wisconsin and never blow out the black tank flush. It doesn't have a check valve at the hook and is self draining. At least mine is that way and I'm going on the 5th winter. It sits outside at the campground all winter and hasn't been covered for the last 2 winters.
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Old 10-21-2018, 08:32 PM   #7
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I've never blown it out either. I'm in North East Illinois. One spring, we had a vacuum breaker failure. Water spewed out of it when I tried to flush the black tank. Seems that some water must have gotten trapped in it and froze...or maybe it was a conincidence.

Regardless, I still dont attempt to blow it out....

TMartin, FWIW, mine was very close to the black tank flush input and very far from the toilet. Hope that helps. Look for an access panel inside the camper above the black water input. Good luck.
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Old 10-22-2018, 03:37 PM   #8
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Originally Posted by tmartin View Post
I have a 2015 legacy and the black tank flush is not working. Iím sure itís the check valve but I have not been able to locate it. My best guess after tracking the lines is behind the toilet but I have not been able to remove that panel, appears to be screwed from the back. That leaves removing the metal tv box for the outside tv to find it. Anyone had to have this check valve replaced and if so do you know where it is? Thanks.
My anti siphon valve failed also, no water would go through. I did clean it out and it sort of works, good enough for now. As to location, mine was at farthest end from dump valve, other side of the unit actually as the tank pretty much goes from side to side and is somewhat narrow. It makes sense that it would enter from the opposite end as it is supposed to flush. Grab a flashlight and keep looking for it at the other end of tank from discharge area. If you can see where a blue pex line goes into the tank it will be close and also higher than the tank itself.
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Old 10-25-2018, 12:57 PM   #9
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Couple of suggestions that may help. Our last motorhome, a Dynamax. The black flush never worked from day one. The valve was installed backwards.

I think if you look outside in your water bay, you will find the valve there. It doesn't look there is much room for it to be in the bathroom except for under the sink. If it's not under the sink, check out in the water bay behind the white board the filter and water connections are mounted on.
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Old 04-30-2019, 10:52 AM   #10
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I finally found the black water flush anti siphon in my 2015 legacy. Itís in the bedroom behind the access panel for the shower controls.
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