1 Empty and thoroughly flush the black tank. Drain the gray tank to flush away any remaining black material. Close the valve.
2 Place a container underneath the valve to catch drips. Suitable containers include an old coffee can or aluminum baking tray.
3 The valve slides in between two flanges. It is held on with four nuts, bolts and washers. Hold the bolt still with one wrench and turn the nut until all are removed--or vice-versa.
4 Gently pry apart the two flanges and work the valve out. If there is resistance, push on the sewer pipes so that they move away from one another. The valve may drop out, or you may need to twist it gently.
5 The valve has two rubber gaskets, one for each flange. Check to see that they came out with the valve. If not, pull them off the valve. There may be some dripping at this stage.
When you remove the old valve, you’ll see the adapters. It’s okay to use the same adapter fittings, but be sure to use new seals. Lightly coat each seal with a lubricant (I recommend Dow Corning’s 111 seal grease) and place them on the lip of each adapter fitting.
Replace the Valve
1 Clean both flanges and apply a light coating of grease to the pipe and the new valve. Place the new gaskets on the new valve.
2 Gently slide the new valve between the flanges. Start the bolts and turn the valve gently to ensure it is seated firmly. Gently tighten, but do not over tighten, the bolts and nuts.
3 Wipe off any remaining mess and discard the rags. Your new valve is ready to use.
# Some valves may have a remote control cable that should be disconnected from the valve before the valve is removed.
# If you tighten the bolts too much, this will compress the valve and the slide inside may not move freely.
I decided to change out all the bolts on both valves. I went with stainless bolts and washers, and used stainless wingnuts on the back side. The stainless guarantees that there'll be no corrocion to make removal difficult. The use of a wingnut allows me to get at least one finger back there to hold a backup while I'm using my power drill/driver on the front side of the bolt. Now, the next time I do this job I expect it will be much, much easier. Glen Kenney 1994 U240 36'
To get the bolts on my black tank, I tack welded a piece of steel line right to the end of the bolt. Slide the nut right down it then pushed it on the threads and turned it a half a turn with my finger. John S. 2001 U320 42'