1971 K20 4x4 brake problem need advice!

[derick1]

Finally got my 71 K20!!!! but need some help. The passenger side from caliper was frozen, so I replaced calipers, pads, an rotors on oth sides. That problem was solved. When I stop it stop evenly on both sides but just really suck. Pedal is hard to push and still seems like the stopping power is not there. Was thinking its the power booster, master cylinder, or maybe rear beaks? I was leaning to the rear breaks but Im not sure any Ideas?