1997 F-150 brake problem

1997 F-150, 2 wheel drive, disc-drum set-up. Replaced front brake pads, and found left front caliper seized. Replaced caliper, now can't get the air out of the system. I've gone through over 3 quarts of brake fluid and get foamy fluid coming out of the caliper. I double checked all the connections and lines - front and rear lines including the rear wheel cylinders. No problems found. I have braking power at the bottom of the stroke and get fair pedal after pumping the pedal about 5 times. I know that is air in the system, but where is it coming from? Could the master cylinder be bad with the seized caliper? 3 quarts of brake fluid is enough to do several brake systems. Any help would be great.