dodge dynasty brake help?

[n229q]

i replaced my front brake pads a few weeks ago. when i started to push the brake pedal to pressure the brakes after a brake job, the pedal went to the floor and the brake light came on. after a few pumps, it went back to normal. after a week or 2, i was driving trying to stop at a light, the pedal went straight to the floor with the brake warning light on, again. pumped it a few times went back to normal. now this weekend when i drive, i have to pump the pedal 4-6 times to get pressure to stop the car, because once will not work, it will go to the floor. i checked the calipers in the front and the drums in the rear, no leaks whatsoever, as well as check the lines for air in them, though i don't see how it could have happened this way no lines were open prior to the brake job. checked the master cylinder for dot3 and has enough, even though the brake job was done, it still had fluid. im leaning more toward the master cylinder itself, maybe it blew the piston when i pressed the pistons back into the calipers when i replaced the pads. anyone agree with this, any suggestions. it goes on a 1989 dodge dynasty LE. all braking equipment is oe.