Which master cylinder bore size to use for power disc Kelsey/Hayes

But also sounds like something the matter with the front brakes. Maybe it'll clear up once you have a good master cylinder and a proper bleed job…but could also be poor choice of brake pads, brake pads improperly bedded in/glazed, one or more stuck pistons in the calipers…

As far as what master cylinder to use, Dana67 is correct. Make sure you use a disc brake master cylinder, not a drum brake one. Such as this one.