Probably had exhausted the source of fluid after drawing down the aft lines? Then you're getting an air pop?
I have only reduced the Master Cylinder of brake fluid, so as not to allow any air bubbles or air infiltration to the lines. With an archaic hand pump, I'd pull fluid through starting with the RT rear line to fresh looking colored fluid, left rear, RT frnt, then left frnt - NEVER allowing the freshly poured fluid to be diminished from eye sight in the Master Cylinder.
Don't know if this helps, but I take no chances with a dry line from the Master Cylinder.
The above was on my 83 300SD.
