Question
The valves on my system will not shut off regularly.  They will work properly, then sometimes they will keep running after the timer has ended. The system is on a well.  Is there to much pressure?  Would installing a back flow valve help this problem?  

Answer
Try adjusting the flow control on your valves.  They may be open too much, which will cause them to close slowly or not at all.  The best way to do this is to close the valve down all the way, then open it all the way, counting the turns.  Then close it down halfway.