Terry - this is Jase, i spoke to you on the phone about a month ago about these problems as i had also sufferred from a leaking pumptec head and was pulling my hair out
Mine continued to leak after i spoke to you, but is now hopefully sorted - i think you have the same problem.
Basically, the pumptec pumps are top quality, they dont just leak after a few weeks for no reason. I explored all the normal routes as you did with just temporary bouts of fixing the leak and then having drips again a few weeks later. I knew something else in the set up was causing this, i couldnt just get unlucky with so many heads/kits leaking.
What happened with mine was the regulator which adjusts the pressure of the pump (on mine its a big brass bolt that screws in and out to adjust and is located on the out hose from the pumphead) wasnt working properly - it was slightly seized. I could still adjust pressure, and i was still getting backflow to the clean tank fine, but the seizing was still enough to mean there was too much pressure building in the pump head. After a few weeks, this would just cause enough pressure to unseat one of the Orings in the head and cause it to leak. I greased the regulator piston and found i could sense the flow had improved, so i then opened up the pump head and found that one Oring had unseated. I then opened up the other pump head i had and found the same Oring unseated.
I placed the Oring back, got a regulator kit to replace the plunger and seal in that and all has been good since - better flow, pressure AND NO LEAKS.
I hope that makes sense - basically i think you need to get the regulator on the Ninja checked/replaced as i would bet my last quid you are getting the same as i was - too much pressure build up in the pump head pushing a seal out of place and causing a leak shortly after.
Its not your pump mate, dont keep getting new heads/pump head seals, it wont work