Pedal goes to the floor?

Hey guys, I'm not a brake guy so any advice would be great. I've got a 74 duster, and when you push the brake Pedro it goes to the floor. I've put on a new master cylinder, and new front calipers trying to fix this. I've bleed about a gallon of brake fluid through them too. Doesn't seem to push anymore air out, just fluid. With the car off you can pump them up a little and peddle will finally stop about 1 inch of the floor. But with it running and the booster helping it will go to the floor every time. I've jacked the front up and with the peddle flat on the floor the front brakes are working enough to hold the tires from turning, but only when it's flat on the floor. When driving it, it seems to only have rear brakes, and if you aren't careful you will lock up the rears. Like I said, I'm not really a brake guy, so any help here would be great.