Just had a thought(just the 1) have you bled the oil system? i.e. fill the sump, coil lead off,return oil line into a container and cranked over, more oil crank over, more oil until you start getting oil through the return hose, bit like bleeding a water system on a regular car
I havent bled the system but I did fill the oil fillter and crank it over untill i got the green oil pressure light, I'll bleed it through when I get home, Like you, I did think air lock but thought that with the oil beig under that sort of pressure it wouldnt be that!
So, Ron....when did your oil leak start? Has your motor been together long? Has it always had a leak? The only time I ever blew up an oil filter or two was on my old sandrail. The oil pressure relief by the oil pump was stuck. The case I had at the time had all the oil galley plugs drilled and tapped for pipe plugs. The one by the pressure relief valve got tapped a little deep and the plug came in contact with the relief plunger. By backing the plug out just a touch, the plunger fell right out in the sand. It's extremely unusual to have a problem with an HP-1... there's got to be a reason someplace. I'd look at the plungers and springs and use stock OEM stuff.
This all seems to have started with an oil change which I did at the same time as it started to get colder here.
I'ts a all new engine thats done about 3000 trouble free miles that I have been really pleased with. There has always been a small leak oil around the inlet hose to the filter mount that is now cured , that was nothing big just a pain getting oil on the muffler.
I know its something that I 've done but cant figure out what, as it was fine before.
It only blows the fillter when its been cold outside 5 c or lower. If it does start without blowing the fillter or gasket its fine on tickover but the pressure seems to blow the oil through the gasket on the filter on a run regardless. Pressure wise it shows 25 psi on tickover when cold and 12.5-15psi on tickover when hot. On a run its about 30psi cruzing and 40-45 psi when gunning it. I did see it spike on the pressure gauge once back in the summer upto 70 psi but I've never seen it do that again.
I'll try bleeding it through as Ben says and see if that cures it check the hoses for kinks etc if all fails then it sound like it time to drop the oil and get the plungers out