The diaphragm is leaking or the seat on the diaphragm is not sealing probably not a solenoid problem. Get the info off the brass valve and I can get you to the kit. Probably less than 10.00.
As an option you can quickly crack the valve open by removing the four cap bolts and clean the valve. A little junk can hang the valve and make it not seat tight. This appears to be a direct acting valve that opens and closes from the solenoid action., 10 min's if you have the part.
Goyen is a good company, I used thousands of a different style diaphragm valve on dust collectors I built over the years.