Pull the bypass valve plug, spring and piston out and clean them up, do the hosuing bore also It, the valve and piston, needs to slide in and out with no tapping or forcing, I've seen that in the past . One sure way of telling is to remove the pump and bypass plug and turn the pump so the bypass valve and piston are pointing down, if the piston slides out and the valve stays in the pump housing clean it out It only takes a tiny bit of grit between the pump housing and valve to make it hang up in the bypass position, especailly when new