How do you bleed the brakes in a 1977 Motorhome / RV?

I have a 1977 GMC MIDAS motor-home, and I am having a hard time with the brakes. It has 4 disc brakes. I changed the front pads, and calipers on both sides. I tried bleeding the lines but the pedal is still really soft. I have to push the pedal all the way down until I feel brakes, and that's with new pads and calipers. The rotors do not looked warped. I read something about turning the wheel to both locks to engage the pump or something? I tried everything, I think it may have a brake power boost system. I did add brake fluid to the master cylinder because it went down after the bleeding process, which I hope was what I was supposed to add. Anyone have any ideas, or somewhere that the proper bleeding process is described for this vehicle?