2002 Dodge Caravan confusing brake problem. Air getting in

Have problems with brake pedal going way down. Bled brakes and found air in left front only. (Lots of air.) Continued happening every couple days. Found what looked like a leak at master cylinder. Changed master cylinder. This did not help. Changed left front caliper and brake hose. There is no sign of leaking fluid, but I keep getting air in left front. Van does not lose any noticeable fluid in reservoir. Cannot get pedal to firm up solid. Will get better after bleeding, but always has a couple bubbles every time I crack bleeder. When I do bleed it I pump up the pedal and crack bleeder. It takes 4-5 times before I get hardly any fluid. A lot of air getting in somewhere.