Tuesday, November 28, 2023


Ask Dave: Can I use mineral oil to lubricate my RV’s dump valve?

Dear Dave,
My RV’s waste tank valves are starting to stick. A fellow camper suggested mineral oil to lubricate the dump valve. What are your thoughts on lubricating waste tank valves?  —Charlie

Dear Charlie,
I do think it is a good idea to occasionally dump some recommended conditioner down both the black and gray water tanks to lubricate and condition the rubber seal that the spade valve slides in and out of.

The Valterra gate valve is the most common model used by RV manufacturers. As you can see in the photo, it has a plastic gate or spade that slides in and out of the dump hole. There is a rubber seal on each side, which should be conditioned at least at the time of storage and a few times during the camping season.

So can I use mineral oil to lubricate my dump valve?

A lot of “old-timers” use mineral oil or even cooking oil, like vegetable oil. However, there are products made specifically for this purpose. Thetford has a great product we have used for several years called the Drain Valve Lubricant. They recommend using 1/5th of the bottle with 4 gallons of water. Mix it up by driving around some, then open and close the valve a few times keeping as much mixture in the tank as possible, then let it sit overnight. This cleans and lubricates not only the seals but the plumbing as well.

A bottle of Thetford Drain Valve Lubricant

Another area that you will want to inspect is the actual lever rod and cable if you have a remote location for the handle. Many trailers will have the handle on one side and a cable running across to the tank. This can get kinked or rusted and make it hard to open and close the valve, as well. This photo shows the drain pipe bayonet with the lever just to the left and a long rod dangling with little support.

It is recommended to occasionally lubricate the rod with a dry lube such as 3-in-1 spray or CRC. Do not use WD-40 as it will remove the lubricant that came with it and is inside the housing. Extend or open the rod and valve and apply the proper lubrication to the rod and run it in and out.

One other item to inspect is the proper installation of the cable, if it applies to your vehicle. The cable should be secured within 24” and have 6” of straight cable coming out of the valve housing. See the detail from Valterra’s Installation Sheet to make sure your cable is installed properly or has not loosened up from a secure area.

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.

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Dave Solberg
Dave Solberghttp://www.rv-seminars.com/
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. He has been in the RV Industry since 1983 and conducts over 15 seminars at RV shows throughout the country.



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Tom (@guest_174328)
1 year ago

If you dump cooking oil, or another oil of that type, you aren’t accomplishing anything if there is any water in the bottom of the tank. In order to be effective, the lubricant needs to be in contact with the rubber in the valve. Oil that is just dumped into the tank while there is any water also in the tank will just float on the surface of the water, like it does in a jar of oil and water on the counter at home. My thought is that products like the Thetford one that is mentioned, are a compound that is actually heavier than the water, and they probably sink to the bottom of the tank and collect at the seal.

I haven’t actually tried this by putting a small amount in a jar and letting it sit, but I think that I will the next time I use some.

Bob (@guest_174321)
1 year ago

Any petroleum based lubricant can cause seal damage, cracking and swelling. One of the commercial seal lubricants, or silicone spray is the best way. One tip I did see is to drill a small hole above the handle and spray silicone into the hole while moving the handle in and out. This will get the lube on both sides of the seal and the rod.
Plug the hole with a small screw. Just make sure the tanks are empty!

Joe (@guest_174361)
1 year ago
Reply to  Bob

I drilled the hole and spray silicone in it a few times a year and also on the metal rod. What I also do after rinsing the black tank is open and close the black valve while dumping the gray tank, the soap tends to help lube the black valve seals.

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