Just a guess without being there, but check under the solenoid plunger to see if there's something keeping it slightly open. Look VERY CLOSELY, it doen't take much!!!
The working pressure in the manifold, when one of the other valves is open, might be just off enough to keep the plunger from closing all the way and sealing the port...
Good luck!!
The Irrigation Specialist Mfg' Az.
Please check my website and you'll find what I do and the unique tools I make and market.
Real timesavers, especially the Suck-Tube and Krik-It.
http://www.tismaz.com