oil pressure gauge failure?
There should have been a cotter pin in the end of the relief valve port, it holds the freeze plug, spring, and plunger in place.
Without the pin, vibration can work those parts right out of your pump.
\'71 Torino GT 466/C6
11.3:1, Edelbrock CJ RPM heads, 250/262 @ .050 solid flat tappet, Weiand CJ stealth, Holley annular 850 DP, Hooker 6115 headers, TCI streetfighter converter, C6, 3.89 31 spline trak-lok.