Is this a low head, lower than other heads on the same zone? This isnt something that would occur, barring any earthquake activity, rather it either is or isnt a low head. The seal-a-matic references a small rubber seal (check valve) in the bottom of the sprinkler head some 570's have these as well, check-o-matic is what Toro calls it, this check valve, if present, could be warn, damged or have debris on it that causes the head to weep. Also you could have a zone valve (electric valve) that is weeping and causing this head to seep. Before you go replacing electric valves I would recommend investigating it further, taking the head apart, etc..