The bypass valve in the pump is supposed to open when the spring that controls it is overcome by oil pressure. A common trick before the advent of different springs, was to shim the spring to achieve higher pressure from the pump. 80 psi is not excessive pressure from the pump (20W50 in my 383 gives me this pressure cold), but I bet that the pressure is much higher at the oil filter when it blows, due to the design of the system. The common port for reading oil pressure is on the back of the lifter galley, and that is a looong way from the pump. I make a habit of disassembling the pressure relief valve before installing the pump, so that I can confirm free movement of the valve. The last Melling HV pump I purchased had a burr on the valve, which would have spelled disaster for my engine, had I started the engine without checking it first. I am willing to bet that this is what happened here, and the fix is simple, and easy to carry out without disassembling the engine. If there are no problems with the valve, and no trash in the pump to wedge it in place, and defeat its function, steal the pressure relief spring from your old pump, and use it. You might have the wrong spring in there, as the person that assembled your pump was likely not a highly-skilled, highly-paid technical type.
