RV Holding Tank Valve

I have a 1974 Prowler trailer. I think the black water valve leaks liquid because only solids remain after several days of use. I purchased a retrofit valve but on inspection I don't see how this new valve can be water tight. The slide in the valve does not touch the rubber seals all the way around. On installation do the seals compress and contact the slide valve or do I just have a defective design? My trailer is in a remote area so I want to make sure I have all my ducks in a row before I tackle this nasty job. Your advice is very much appreciated. Thanks.
Pat, (Carrollton, TX)

Pat, the termination valve itself is but 1/5 of the total package that ensures a leak-free termination assembly. As you’ve noticed, the slide itself appears to not reach the top or bottom of the valve. On your waste drain plumbing, you’ll find two adapters which sandwich the valve between them. It’s this sandwich, with the seals on the two adapters that create the leak-free seal. The valve itself must be a full-way valve, meaning the valve must have a full three-inch diameter when fully opened. Therefore, the seals must be located beyond this 3-inch diameter, hence the adapters on each side of the valve.

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. Taking care not to disturb the seals, carefully insert the new termination valve between the adapters and secure with the four bolts. Snug all four bolts down finger-tight and tighten them in diagonal corners. Take care not to over tighten them. Do not crack the adapter fittings around the bolt heads or nuts.

But since you are going to this trouble, you may want to consider installing a 12-volt electric waste valve instead of the manual type. Check out this valve! Having the electric gate valve will enable you to open and close it from inside the RV with the simple push of a switch. It installs in the same manner as above. Take a look and see what you think.