Replacing and maintaining the valve seal in a Dometic 310 or 320 RV toilet assures that noxious odors will stay in the holding tank. The process is simple but you have to get over the “icky” factor.

By Bob Livingston / Photos by the author

t’s a dirty job, but someone has to do it. Maintaining an RV toilet is not the most desirable chore — but let it go too long and you’ll find yourself searching for the nearest bush. Beyond basic cleaning, the most common malady is a dried-out or damaged valve seal which prevents fresh water from standing in the bowl, ready for the next use. When operating properly, after flushing the toilet water is allowed to flow into the bowl, providing a natural seal against odors coming out of the holding tank. When it doesn’t happen, the lack of water can lead to obnoxious odors seeping past the valve seal — and it doesn’t take a lot to fill the relatively small quarters in an RV.

Over time — sometimes shorter than expected — the valve seal in the Dometic 310/320 toilet will become damaged from hard water deposits and crud that was not flushed properly, not to mention paper becoming logged in the valve (again, from poor flushing habits). It also can become abraded from hard-water deposits that have been allowed to build up on the valve. Fortunately, this seal can be replaced without removing the toilet, although it’s not a pretty job.

Replacement seals can usually be found at RV supply stores and online. It’s best to keep a spare in your RV; during the supply chain back-up, these seals were a little scarce. They are not cheap; a Dometic-branded seal will cost upwards of $37 off the shelf (around $29 at Amazon). Generic seals are available for less than half price, but we have not used them, preferring to stick with the Dometic seals which have always fit in the opening without undue effort.

There are two ways to accomplish a seal replacement: You can simply pull out the old one and put in the new seal — or you can take the time to do it right. We subscribe to the latter.

The first step is to turn off the water source and clean the valve thoroughly. Oftentimes the surface of the valve will be caked with calcium that should be removed. A good dose of Lime-a-way will dissolve most of these hard-water deposits. Once the valve surface is smooth to the touch, hold the flush lever down and stuff some paper towels into the hole. You can use an old microfiber or cotton cloth, but paper will eventually dissolve if it accidentally makes its way into the holding tank. This will catch anything that is accidentally dropped — including the seal — into the pathway to the holding tank. Foreign items that are allowed to get into the holding tank can create misery later.

Removing the old seal is, well, icky — there’s no other way to describe it. As gross as it seems, however, you may not want to wear gloves because you’ll need a good grip to maneuver the seal. (Of course, remember to wash your hands with hot water and finish with sanitizer.) After removing and discarding the old seal, the area around the opening was cleaned with a rag and paper towels. Then a putty knife was used to remove any calcium build up on the valve. If you are not proficient with a metal putty knife, consider going to plastic, although it’s not as effective. Also, if you think you might let go of the putty knife while working on the valve, tie a string through the hole in the handle and the other end to your belt or stationary object. It takes some patience to remove all the calcium, but it’s important that the valve does not act like sandpaper when opening and closing. In the end, the leading edge of the valve should be smooth and conform to the original curvature.

The valve is closed before installing the new seal. To seat the valve, it’s just a matter of getting the rubber to tuck in around the hole. Usually, it takes only a few minutes to accomplish this task, but if it resists apply a dab of plumber’s grease to seat the rubber. Open the valve to make sure the seal is fully seated, remove the paper towels and proceed to coat the top of the valve and seal with plumber’s grease. Use a thin layer of the grease; more is not better. We’ve had great luck with Oatey Plumber’s Faucet and Valve Grease, which is available in most home improvement stores for around two bucks. The 1-ounce tube will last a long time.

There’s no getting around the fact that hard water will play havoc with toilet seals, valves and water ports for the bowl. Cleaning these areas frequently will prevent calcium build up and using Lime-a-way every couple of months will also extend the life of the seal. Consider coating the seal and top of the valve monthly with plumber’s grease. Your toilet, RV’s occupants and guests will thank you.