I have a 75 25' Airstream and I bet the two trailer's construction is just
about the same. Sounds like someone added the third valve. I just replaced my
valves on both the black water and the gray water tanks. Its a hard job because
you have to cut out the bottom of the trailer and saw out the plastic drain
pipes and get new valves. You have to get under the trailer to do this job.
Material costs about $45.00. When the new valves are replaced you then just have
to get a new piece of aluminum and close the bottom of the trailer back up.
Sounds like the rubber seals on your valves are just worn out. Its not an easy
job. jauman2346@aol.com