Help with Aprilaire humidifier -- not working?

We discovered this week that the humidifier attached to our furnace was not working. We checked the power- working. We checked the water line- turned on. So we drove 40 miles round trip and spent $80 for a new solenoid. Installed it and checked for water and electricity and that the unit was actually turned on and still no water coming through. The place we bought the solenoid will happily come out and replace the whole unit for $700! Help? Ideas?