If you are sure that the water is definitely leaking at the seam as you say, then maybe the cap is cracked.?? But the likely hood of that with multiple heads does'nt seem right. Sometimes leaks appear to be from one area but are actually originating elsewhere. When the system is operating, can you easily push down on the riser? This could mean lack of pressure. When heads first come up they are designed to "flush" water out between the riser and the seal and then "seat" after a few seconds. If there is not enough pressure under them they will continue to flush. By the way, the PGP head does not have a filter basket as some other heads do. If you take the guts out and look underneith it there is a filter screen within it. That should be clear. From your description, check loose cap,verify crack somewhere, insufficient pressure (which could come from anywhere in the system). My guess would be a water flow/ pressure problem.