When you replaced the diaphragm and the solenoid did you use the same parts? You may have debris stuck in the valve causing the diaphragm not to seat properly. The other thing but highly unlikely is you have a voltage leak to that valve. I would first flush the valve out to make sure there is no debris and then check to make sure the solenoid is not cross threaded. Also there is a little rubber O-ring under the solenoid make sure that is not missing.