My first thought when you described your situation was "bleed screw". When you look at the top of the valve is there a cross looking handle right in the center? If there is, that is the flow controll and is probable a Weathermatic which has 8 stainless steel screws like you mentioned. The bleed screw, which is usually a small black plastic screw, allows you to relieve the pressure on the upper side of the diaphram which will allow the system pressure to open the valve. If the screw has been damaged or has been loosened it will leak into the box and your zone will run continuesly. Sounds familiar. If possible, turn your system off, see if there is a slot in the top of that screw. Either standard or phillips. If so, see if you can remove it so you can check it's condition. By the way, has anyone other than you ran that zone manually recently? When I say manually I mean activated that zone from that valve itself? I am not saying that is the problem but, that is the first thing that came to my mind.
