Sunday, January 29, 2023


RV black water flush sends water under the shower

Dear Dave,
I hooked up my black water flush and found water coming from under my cabinet. I removed the inspection cover under my shower and could see water on top of my black tank. Any ideas? —Terrance, 2016 Grand Design 308BHTS

Dear Terrance,
Most 5th wheels have the black water tank in the middle or offset, so the flush valve you actually connect the hose to is not directly connected to the tank. Rather it has a hose going from the connection to the actual spinning valve on the tank. So you have a variety of areas that could leak when connecting pressurized water, either directly behind the service center connection, somewhere along the hose going to the black tank valve, or at the connection of the hose to the valve. This is the connection on a Keystone Raptor 2016 that is mounted on the sidewall but also has the line going to the tank.

Most owners do not think about this flush valve when winterizing and there is often water sitting in the hose from the service center to the tank. When it freezes, the hose ruptures and you have a leak. You should be able to identify if the water is fresh water or black water with a test kit from a home improvement store. My guess is it’s fresh water spraying from a crack in the supply line.


Ask Dave: The toilet leaks from underneath when cleaning black tank. Why?



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Jim Tilton
22 days ago

Should you regulate the water pressure on hooking up the black flush connection since I’m sure it’s the same piping as rest of the camper to keep the line from being over pressure ?

27 days ago

In our grand design 5th wheel the vacuum breaker is behind a panel in the closet. Four screws and a bright light and you can remove the panel. Vacuum breaker and shower valve access are both accessible. The comment about the seal sticking is probably correct and can usually able to be unstuck without replacing the vacuum breaker.

27 days ago

As Jim calls it a vacuum break I think can also be called a one way check valve. I’ve replaced one each on two campers. Not a big job doing the replacement but it’s sometimes difficult getting to the valve through all the other pipes.

Jim Johnson
27 days ago

We have a 2017 bumper tow 34′ Keystone. I have an alternate answer to the issue. The vacuum break for the black tank flush line. Keystone locates this device high up just under the bathroom sink. The device is made of cheap plastic. We thought our first failure was due to freeze and I replaced the device. When it happened again after sitting for several months with no exposure to freeze temps, I figured out the real problem.

After being in storage the water pressure driven flapper tends to stick in the ‘break’ position. If you turn on the water too fast to feed the blank tank flush, the water will pour out of the vacuum break through the air vent and flood the floor under the sink cabinet and find its way under the shower where the holding tanks are often located.

Especially after sitting for a few weeks, turn on the water feeding the flush line slowly the first few uses until the flapper is again freely moving. I also installed a battery operated flood alarm under the sink.

27 days ago

That is the first line I do when winterizing so I don’t forget it. I have one of the hand pumps to do that. I also pour some antifreeze in the fresh water fill.
The last line I do is the city water inlet. Take out the screen and press the check valve while the pump is running antifreeze.

Bob p
27 days ago

Didn’t read the owners manual!

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