The valve you mentioned might be the stop solenoid located on the back side of the injector pump right above where the injector lines connect. When you turn the key on you should hear or feel it snap. That is what turns the motor off once its running, if its stuck or has gone bad then it won't start.

Are you getting good hard crank from the starter?

have you cracked the injector lines at the injectors to see if they are getting good spit. If so then the stop solenoid is working if not then it could be air or the stop solenoid. Try cranking the engine over with the injector lines loose to make sure there is no air in the injector lines, once you get good fuel tighten them down and try starting again.

Sometimes old diesels like a good "drag" to get them going if they've been sitting. If you have the means tow it behind another truck to try pull starting it.