Product review (5 stars): Easy Fix for my Humidifier!
Worked great for me!  The water from my Humidifier was constantly running and I determined it was the solenoid valve.  I did have to loosen the bolt at the end and rotate the valve 180 degrees as others have mentioned (to have the flow going the proper way).  Otherwise it was a 10 minute fix.  I am by no way a furnace repair tech, but this was an easy fix.  Here is the video to show how to replace if needed:

http://video.aprilaireparts.com/v/1043089725/aprilaire-4040-solenoid-valve/