[G.M.]

John it sounds like the bypass valve in the pump has a weak spring. I have found the springs in the latter pumps and in the front of the engine to cause fluttering and low pressure. I made a sleeve that fits snug on the pump with a gauge and adjustable restriction on the output, remove the drive gear and run the pump with a 1/2" electric drill. You can make a shim out of 1/8" brass pipe for on the spring adjusting the thickness of the shim to get 55 lbs of pressure. I would never put a pump in new or used without testing it. G.M.