I had a clue since the gravity drain from my sump was always wet in the area where it dumped in the back yard. I looked into the sump and discovered water was draining from a PVC pipe which went to the furnace. Further investigation showed it came from the humidifier drain, and since this was August I knew something was wrong!!
Above is a picture of the solenoid that controls the water flow.
I turned off the saddle valve that feeds the humidifier and the water stopped flowing from the PVC pipe. I removed the 24 volt supply wires from the solenoid, then removed the outlet hose from the solenoid and turned the saddle valve on. Sure enough water spouted from the solenoid! For some reason it failed to shut off the water to the humidifier.
Disassembling the solenoid valve I found that the seat in the brass housing looked a bit rough but on further microscopic inspection, determined that it was OK The solenoid armature with a rubber tip moves easily and the rubber looked fine.
After cleaning the seat and the rubber tip with a Q-tip, I reassembled it thinking I might have fixed it but it still gushed water.
I decided to completely disassemble the valve, taking both the stainless steel fitting from the inlet and the brass fitting from the outlet off. I looked into the hole from the inlet side and was amazed when I could see right thru the valve! Here is a picture of the hole inside the valve.
Here is another picture of the solenoid valve with a wire going right thru the valve to show how large the hole was that had been eroded by the water jet.
I called the local dealer who installed my HVAC system and asked if he had the solenoid in stock. Yep, and your cost is only $190.47!!!! I found a universal humidifier valve online for $50 including shipping and will order it soon.Update: November 19th 2015 - After looking at all the replacement solenoids, all of which were brass, I decided to repair the old one. I found a stainless steel 2-56 screw and nut in my junk collection. I wrapped the screw head with teflon tape and inserted it into the hole which was "drilled" thru the brass wall with the heard on the input side. I tightened the nut and tested the solenoid. No more leas! The stainless steel head will last a lot longer than the brass did!