I found out why my Dishmaster had a leak that would come and go. It uses a
threaded bushing to jack a washer on a valve stem against a valve seat. The
washer is not rigidly connected to the valve stem. That means that the
washer is not force to rotate with respect to the seat.
The bushing cracked along the jacking direction. That allowed it to spread.
During use, it could spring open or spring shut with some hysteresis. In one
case, the seal would be made without much pressure. In the other, more force
had to be applied to make the seal.
Replacing the bushing solved the problem.
-- Ferme le Bush