Darrell -

If you are sure there is no external reason for it, then your diagnosis is
probably correct. The seal between the two valve chambers probably opened up.
You might try to gently tighten the screws that go around the valve block.
Otherwise, you have a project ahead of you. Good luck.

Bob Bosche'¶
(I have a Weber Duo-Art Reproducing Grand, and have rebuilt several others.)