this may sound silly--but make sure you have the solenoid installed with the 'I' and 'O' oriented correctly ( in and out ) If the solenoid is backwards it wont seal.
+1. I didn't see an 'I' or 'O' on mine though, so I just tried it out, and found it leaked.
Then I found this picture in a build guide:
And found it was backwards. I used the two holes in the metal solenoid body as the reference to which side was the input.