when there's air seeps inside the fuel pump or injector hoses, if you tighten the fuel filter cap, it makes pressure onto the cap, pushes the cap, that's why the filter cap couldn't be sit onto the housing thoroughly. After the air bled out of the housing, it is so easy to tighten the filter cap! It seems the Racor button on the hand pump of the water/fuel separator is useless, since we didn't feel pressure on it after 100 times push.