Water leaking in rear bathroom when using rear tank flush.

We have a 2016 Sundance 3700RLB, I recently decided to flush both front&rear black water tanks. As I was flushing the rear tank with the flush system, my wife noticed there was a large amount of water on the rear bathroom floor. Has anyone else experienced this problem? Any ideas of what would cause this? I’m hoping not to have to totally dismantle the rear half of the trailer in order to fix this problem.

GarryN2016 said:

We have a 2016 Sundance 3700RLB, I recently decided to flush both front&rear black water tanks. As I was flushing the rear tank with the flush system, my wife noticed there was a large amount of water on the rear bathroom floor. Has anyone else experienced this problem? Any ideas of what would cause this? I’m hoping not to have to totally dismantle the rear half of the trailer in order to fix this problem.

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You didn’t by chance pour a bucket of water in the toilet real fast?

First. Are you sure you didn't over fill the tank with the valve closed and force water up to the toilet? If no then read on.

I'm not familiar with your model, but the black tank flush on most (if not all) tanks have a vacuum break in the line that goes to the tank flush. I have one black tank and the vacuum break is in the corner behind the shower and is accessible by taking off the shower valve. These vacuum breaks are notorious for cracking and leaking when in use. Many have changed them to brass. Here is a picture of what you are looking for. Hopefully it's accessible.

On Edit: You might want to call Heartland service and ask them if/where it is. Have you VIN number ready.

Bogie said:

First. Are you sure you didn't over fill the tank with the valve closed and force water up to the toilet? If no then read on.

I'm not familiar with your model, but the black tank flush on most (if not all) tanks have a vacuum break in the line that goes to the tank flush. I have one black tank and the vacuum break is in the corner behind the shower and is accessible by taking off the shower valve. These vacuum breaks are notorious for cracking and leaking when in use. Many have changed them to brass. Here is a picture of what you are looking for. Hopefully it's accessible.
View attachment 64623
On Edit: You might want to call Heartland service and ask them if/where it is. Have you VIN number ready.

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It could have a flush for the rear black tank, but I doubt it. Our Cyclone doesn’t have a rear black tank flush and that’s why I was asking if he had used a bucket to flush it.

Flick said:

It could have a flush for the rear black tank, but I doubt it. Our Cyclone doesn’t have a rear black tank flush and that’s why I was asking if he had used a bucket to flush it.

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It does have a rear flush in the back black water tank, and no I did not dump a pail of water fast down the toilet at any time. All I did was hook up the hose to the fitting, turn on the water, and my wife noticed that water was coming out from underneath the cupboard under the sink. When I attached the water hose, it didn’t feel quite right tightening it up, my guess is the water line is probably damaged under the trailer.

Silly question maybe. Was the dump valve for that tank open?

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Does your fresh water supply from the park seem to be high pressure? The B&B Moulders plastic Atmospheric Breaker Valve is prone to failure under higher water pressures (that's how mine broke in Wells, Nevada).

Check the 6th bullet point in the first section of this manual for the black tank flusher system: https://manuals.heartlandowners.org/manuals/Plumbing/Tanks/Black Tank Flusher2_Install_.pdf

You might want to look at the results of this forum search. I have it set up in oldest to newest posts. You will see that quite a number of people have had this problem (disaster) over the years. I seem to remember at least 1 owner coming up with a brass replacement for the valve, but I don't know where that info is among all these posts: https://heartlandowners.org/search.php?searchid=7852155&pp=

ON EDIT: I did a websearch for a brass atmospheric breaker valve and came up with this one sold at Lowes: https://www.lowes.com/pd/Cash-Acme-...jGu4IZrtI6MQ0rBAQroaAriREALw_wcB&gclsrc=aw.ds You will also need the PEX tubing adapters in male and female 1/2 inch threads: Male - https://www.lowes.com/pd/SharkBite-...bFrY4pmzVheJmJfTa9saAiwxEALw_wcB&gclsrc=aw.ds Female - https://www.lowes.com/pd/SharkBite-...AySt6fWxZHQfCI4hgPwaApYtEALw_wcB&gclsrc=aw.ds and maybe a 1/2 inch right angle M-F pipe fitting.

wdk450 said:

Does your fresh water supply from the park seem to be high pressure? The B&B Moulders plastic Atmospheric Breaker Valve is prone to failure under higher water pressures (that's how mine broke in Wells, Nevada).

Check the 6th bullet point in the first section of this manual for the black tank flusher system: https://manuals.heartlandowners.org/manuals/Plumbing/Tanks/Black Tank Flusher2_Install_.pdf

You might want to look at the results of this forum search. I have it set up in oldest to newest posts. You will see that quite a number of people have had this problem (disaster) over the years. I seem to remember at least 1 owner coming up with a brass replacement for the valve, but I don't know where that info is among all these posts: https://heartlandowners.org/search.php?searchid=7852155&pp=

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Our water pressure is definitely not high, but there may have been a connection that possibly wasn’t finished properly or possibly a rock or some other foreign object might have pierced the line.

GarryN2016 said:

It does have a rear flush in the back black water tank, and no I did not dump a pail of water fast down the toilet at any time. All I did was hook up the hose to the fitting, turn on the water, and my wife noticed that water was coming out from underneath the cupboard under the sink. When I attached the water hose, it didn’t feel quite right tightening it up, my guess is the water line is probably damaged under the trailer.

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Ok. No problem here. My Cyclone has no flush on the rear toilet black as I had said. Hence, I put in a bucket of water very fast to flush and later found water on the floor. I dried it up and here we are a year later and the water problem has never happened again. I was only hoping your situation was that simple. Have a good one.

That plastic piece of junk they call an anti-siphon valve is most likely the culprit. And they’re not always installed behind a convenient access port, either. Brilliant engineering (or lack of) may jam them up into a wall cavity with no access short of cutting a hole in the wall They also don’t care what electrical equipment they place under them.


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You do not have to replace the antisiphon valve, just bypass it. A lot of us have done this. Just bypass the valve by cutting the water line to it. Splice into the line going to the black tank. Add a check valve into the flush water line just after it enters the UDC fitting. My valve did not work from day one. Someone may have mentioned this but make sure that you use a pressure regulator on the water line to the flush system.

GarryN2016 said:

As I was flushing the rear tank with the flush system, my wife noticed there was a large amount of water on the rear bathroom floor.

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One way this can happen is if you add water through the black tank flush either with the dump valve closed, or if there's a blockage preventing the tank from draining. The pressurized water has to go somewhere and will either go up the vent onto the roof, out the base of the toilet, or it will rupture the tank or separate tank fittings and leak out the underbelly.

Was the dump valve for that tank open and was water going into the sewer while you were flushing the tank?