OK.... lets work through this logically. You have two things here to think about.
1) Why does water always seep from this particular spot? (you know its not because of the head)
2) Where is the water coming from?
So... here goes...
1) Why is this location always weeping? Well, the only real reason for that would be because it is the low spot on that zone. Basically, any water in the line that has enough pressure behind it can push its way up the head (regardless of brand) here and weep out to freedom.
2) Where is the water coming from? Well... once the zone has run its program, the line may be draining out of that location, especially if it is at the bottom of a hill. But, if it is continually weeping, then there must be water coming into the line. The only way water comes into that line is from the valve. So... you have a number of possible things that could be wrong with the valve....
a) The solenoid isn't tightened fully, allowing the valve to be open a trickle at all times
b) The bleed valve isn't closed tightly, allowing the valve to be open a trickle at all times
c) The body isn't closed tightly (very possible with jar top valves), this too will allow a trickle...
d) The diaphram inside the valve has a small tear in it..... allowing...etc
e) Some small foreign object is trapped inside the valve (i.e. small pepple, etc)....
a,b,c are very easy for you to see for yourself. To check for d and e, turn off the water supply to the irrigation system and open the valve up. There will be a rubber membrane (diaphram), check it for small holes... then look inside the valve body for pepples or other small foreign objects.... remove any that you find... put the valve back together and see what happens.
One simple test to see if the valve is still leaking (other than waiting for a bog to form around the head) is to turn the water to the irrigation system on and watch your water meter for movement( of course, nothing else should be using water). Even the slightest trickle of a leak will show up on the meter if you wait.....