Saturday, April 1, 2023


Ask Dave: Why does the gray water tank still leak after replacement?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses a leaking gray water tank.

Dear Dave,
My 2019 Grand Design Reflection 29RS has had repeated problems and multiple attempts to repair the kitchen gray water tank leak. Both certified and non-certified mobile or shop techs as well as the RV dealer have attempted to repair it. The gray tank has even been replaced by a new tank supplied by the manufacturer. The last repair covered by Grand Design was by a dealer using epoxy.

I have filled and drained the tank many times and exposed the underpan to try to locate the source. I can’t find the problem. Is it advisable to use a blowout plug to pressurize my plumbing system with air below 50-60 psi to see if any air or hissing can be heard or found? Or do I need to buy a special fitting as advertised by the FMCA? —Michael

Dear Michael,
From everything I have seen on the FMCA site and from the late RV Doctor, Gary Bunzer, the special fitting that you are referring to is a screw-in adapter with a shut-off valve and pressure gauge. It pressurizes the fresh water system, which includes water lines, water heater, and water pump but does not pressurize the gray water tank.

One reason why holding tanks leak

Typically what happens with most leaks in holding tanks is they flex when getting full and pull apart at corners and seams. Patching typically does not work well. If you had the tank replaced under warranty by the manufacturer, I would recommend contacting the servicing dealer and Grand Design to get it documented. They need to make sure this is corrected to last.

I don’t have much faith in pressurizing the tank. It can be messy and you probably have more than just the kitchen sink dumping into it. If you choose to try it, one way you can pressurize the tank is to use a dump valve cover that has a smaller garden hose fitting.

Air compressor fitting

You can get an air compressor fitting or make your own out of an old garden hose and air compressor fittings. Cut off the female end at about 12”, insert a male air compressor tip and tighten with a worm clamp. This is one I built for blowing the fresh water lines out. You would want to replace the male hose end with the female to match the threads on the cover.

Dial the pressure down to about 20-30 psi and open the valve to the tank you are having an issue with. You will need to plug the drain of the sink. However, as I stated earlier, I doubt just the sink dumps into that tank. You need to be very careful with this method. Make sure the tank is empty, otherwise you will have a mess!

At the factory, we used to do a flooded test on both the black and gray water tanks. That’s where the tanks are filled to the lowest point, which could be the shower in the gray water case. Once the tanks are filled to the lowest point, the tanks flex. If there is a leak, it is easier to find with fresh water in the tank. It could be the drain pipe from the kitchen has slipped out of the opening in the gray tank.

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Grant Edgar
1 year ago

DO NOT apply high pressure to the holding tank. At least not more than a few psi. If you do and the tank explodes it could be fatal.

1 year ago

Trying to pressure test the tank is futile. All the sink and shower drains would have to be plugged and the vent line plugged. Sounds like the leak is not actually in the tank, but a leak in the drain line running to the tank and the water is following the line to the lowest point, the tank!

Bob M
1 year ago

If you have access to the tank and plumbing. When you add pressure to the gray tank and lines. Spray it with a water dish soap solution. If your lucky you’ll see soap bubbles where it’s leaking. Be careful with the air pressure.

1 year ago

The tank is also vented, so you will need to take that into consideration if your plan is to “pressurize” it.

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