The relief spring is there to prevent the drive gear on the distributor or the tang on the end of the distributor shaft from shearing off due to excessive oil pressure, especially with a cold engine. Most Melling pumps come with two springs- - - -at least the small block V-8 pumps I used by the dozen all did. The fact that your pressure is dropping to near zero with a hot engine tells me you've probably got excessive clearance somewhere. If the cold oil pressure was also low, I'd suspect a weak spring.
The fact that oil pressure builds up at all indicates the pump is moving more oil than the engine needs at the time. If one drop of oil was supplied by the pump to the bearing every time a drop dripped back into the pan, there would be zero pressure. As long as every bearing is supplied with exactly the right amount of oil that it needs, and no more, an engine could be lubricated, theoretically, with no oil pressure at all. The pressure is simply there as a safety factor. It's virtually impossible to accomplish such a feat since there's no guarantee every bearing surface in the engine will get adequate lubrication unless an excess of oil is supplied. In my personal opinion, any pressure above about 30 PSI on a hot engine is simply energy being wasted to turn the oil pump that could be used to drive the vehicle.
I'd suggest checking the relief valve in the base of the pump to be sure it slides freely, and be sure the spring holds it closed firmly. I can't think of a way to check the opening pressure of a particular spring without running the pump against a specific engine, because the volume the pump is required to flow against the bearings, oil filter, and other lubrication needs will directly affect the operating pressure. Just be sure the valve is free to move. A friend decided he needed lots of oil pressure, so he put a stack of washers behind his relief spring- - - -so many the valve couldn't move. The engine would stall from the power required to run the oil pump. When he cranked the engine up and went to full throttle (on a 400+ HP race engine) the oil filter split from top to bottom. I would guess he was getting 300 PSI or more of oil pressure on that one. I happened to be standing right beside the filter when it let go! On an open-wheel dirt track car, that meant I got a bath in 50 weight racing oil!
Jerry
