I hate it when things like that happen.....
Just as an aside, why prime the system anyway? If the engine is freshly rebuilt, it will be full of assembly lube, won't priming the system just wash this stuff into the sump? By the time you get the engine ready to start, the oil pressure will be back to zero and it, and the assembly lube, will be back in the sump. Would it not be better to prime the pump and fill the oil filter, and leave the assembly lube to do it's startup job??
(any theories on this would be appreciated as my rebuild will be ready to run in a few weeks and I am in two minds as to what to do)