1998 Dodge Caravan air in the brakes lines

I have had a reoccurring brake problem, every 3 to 4 weeks I need to bleed the lines of air. First there is a click in the the brake pedal. the pedal goes down a little over the next week it ,then it goes to the floor and you need to pump the brakes to stop. that when the brakes need to be bleeded. it works for 3 to 4 weeks then the same thing happens again. this problem has been on going for 8 months. I had a complete front and rear brake job [calpers also]done plus 3 master cylinders, 4 rear wheel cylinders. runs great for 3 to 4 weeks till air reappears in system. Shows no leaks anywhere. I've had 4 machanics go over this problem for months now. every 3 to 4 weeks, first the click then 1 week of pedal pumping then air!!! HELP!!!