Pressure valve inside pump is occasionally getting stuck in the bypass mode, Usually from dirty oil or the an over-honed bore. I’ve had the same issue with a rebuilt pump as you did. I replaced the valve , flushed the fluid but kept loosing pressure from time to time. I finally rebuilt my original pump and problem went away.
And any spec of dirt can cause the valve to stick. I once found a hair from my German Shepard in the valve bore causing a no pressure situation. Found out the dog liked to nap under the engine. Also cleaned a thick mat of hair from the lower radiator core.
No more sleeping in the garage for the old girl.
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Ed
1968 GTO (Thanks Mom)
2006 Silverado
2007 Cadillac SRX
2015 Chevy Express 
Last edited by OG68; 07-28-2019 at 01:57 AM.
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