brake conversion trouble

i know its not mercedes but today a friend and i converted the single piston calipers on his daewoo musso, i belevie the engine is mercedes , to the two piston calipers from a mitsubishi montero sport, so we finished, new brake fluid, new brake lines, pads, rotors ect... and when we went to try them the pedal goes to the ground, so we bled again and again anda again , and no air, the thing is that sometimes they work perfectly but most of the time the pedal goesto the floor, can it be a booster problem??

need help guys

thanx in advance