When the high pressure fuel pump in the plane is running, there are no bubbles between the tank and the pump, no bubbles in the fuel rail or lines between the pump and the pressure regulator, and there are bubbles in the return line between the pressure regulator and the wing tank.  As a separate test, I connected a fuel pressure regulator to the RX7 donor car’s fuel tank which was filled with fuel and had the original submerged fuel pump in it with a clean inlet sock.  Running the pump in this setup gave the same results:  bubbles in the return line, but nowhere else.