To show that much pressure with just the starter motor spinning it tells me it's a serious blockage somewhere and I just can't see the relief valve causing that but I've learnt to never say never in automotive work. Hope you find it soon because it sure has got me wondering!! I would love to know what the exact pressure and RPM's were. I've seen many filters hold over the 100+ mark without leaking!!

94 P/U Ex-Cab 3.0 Automatic 4X4