My brakes feel normal until the engine is started. Then the pedel gets very mushy and is easily pushed down another 1-2 inches. This started suddenly when driving. Glad I had a manual tranny to help slow down. I have replaced both the master cylinder and booster with used items and nothing has changed. Is there anything w/ the ABS unit that could do this? No evidence of any caliper leaks and not loosing any fluid in the resevoir. I've put a vacuum on both of the boosters I have and they seem to hold until I pry out the oneway valve. Air leaks back in once the valves are pulled out but I don't know if this means anything other than proving the valve's OK and there's no booster seam leak.