It is actually common to see this happen.....tighten the screws. The screws on these antisiphon type valves work loose due to movement in the valves when coming on and going off as well as (my opinion here, not a scientific fact) expansion and contraction of the plastic valve parts in sun/shade heat/cold......you will have to tighten screws often on these valves if you want a completely leak free enviornment.
If it is practical...install a PVB or whatever is code in your area for backflow prevention...and install new below ground valves. Would be better.
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