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How much was the plunger(you called it a piston) moving? Sometimes the button falls off the bottom and it moves, but not enough to create the right pressure.
Better yet, pull the petcock and thread in a pressure gauge. You should have about 5-7psi with the motor off. With it running it should be 60-75psi. This isn't an end-all test, they could be ok at idle and not at full throttle.
Yes, I would swap filters(maybe don't throw the current ones away)...