[rheavn]

Dick,
My info is only good if in 2007 Newell was still using the Dupree dumping valves.  They use a round rubber seal that swells and becomes deformed over time and fails to seal.        I replaced all three on my coach.  Your main valve is the one that is closest to the outlet.  Don't know if you plan to do the job yourself or have it done.  Either way it it is a relatively easy job once you have the needed part(s).  First time you can change out the seal assembly in an hour or two.

Short term fix:  May or may not work.  Drain the tank!  Remove sewer hose.  Try applying some silicone grease to the seal when it is open.  Cycle cylinder up & down a few times.

The fix:  Drain the tank!  Remove the two air lines & mark them for replacement.  Remove two screws on top surface of valve.  One of the screws is visible in picture.      Pull entire air cylinder straight up.  On the bottom is the gate & seal.  The seals are a bugger to install so I would advise just screwing on a new gate & valve assembly.      Reverse for installation.  I used a little silicone sealer on top edge to insure no leaking.  

I replace the seals 6 years ago & have had no problem since.

Entire assembly:  http://www.dupreeproducts.com/ProductDet...=2100P-070

Housing:  This part is glued in and is NOT removed.  http://www.dupreeproducts.com/ProductDet...=01000-027

Gate & seal assembly:  This part will solve your problem.  http://www.dupreeproducts.com/ProductDet...=1100P-025

Let us know how it goes.