97 brake problems(loss of pedal)

97 lumina ,76K with brake problems,I have had problems with losing my pedal pressure.(goes to the floor,not spongy) My mechanic has bled the brake lines to get it back. this has happened many times. He has went over EVERYTHING. My brakes are fine!(All tires removed and checked)Air seems to be getting in the lines somehow.The car doesn't have ABS.I have also replaced the master cylinder 3 times over a 3 year period,the first one in 2001,and the most recent was about a week ago.warranty is expired so I can't go there. It seems to happen alot when the outside temps change drastically. There are no leaks. mechanic says they haven't got a clue...they could be ok for a week or a few days..Help!