Probably the wrong master cylinder.
If the bore is too big the pedal will be hard as a rock.If they are manual brakes find a master cylinder with a bore no larger than 1". I just went through this last summer. A 1.125 bore MC made my pedal rock hard...going to a 1" bore cured it.
Another just FYI ....drum brakes have risidual valves to keep the springs from forcing fluid back to the master cylinder, they must be removed when you go to disc. I think they are usually in the master cylinder itself but I think some had them in the distribution block...if they are left in they can make the calipers drag the rotors