Humidifiers and Dehumidifiers - aprilaire 760 leak

My 760 (3years old) started leaking water out of the bottom of the unit where it latches together. The main drain still works(I see water thru the tube)and the unit cycles off and on with furnace. Every time it comes on it starts to drip out of the bottom and collects a puddle on the ductwork. I checked outside and it looks like a few gallons is pumped out every hour so 99% of the water does seem to drain properly. Any ideas? The filter was replaced in Oct2005 by contractor. I pulled off the cover and didn't see any condensation. The filter frame and water ducts I cleared out(not really in bad shape at all). The filter appears to be correctly installed, red dot up, water feed directly centered. Can a part fail to make it overflow and not drain fast enough? Any help please!

The hole in the orfice has gotten bigger. Turn the sht off valve down until you can barely here it. The orfice is in the black tobe coming out of the siliniod.