If the later [ 1982 ] discs have larger pistons, then you may need a larger m/c than 15/16". Obviously, the larger the caliper piston diam, the further the m/c piston has to move [ pedal travel ] to provide braking. You may have a mismatch of parts.
Not even close. The 2.75" pistons calipers work great with a 15/16" master cylinder. Been there, run that, nothing at all wrong with the combination and it is an IMPROVEMENT over stock A-body 2.6" pistons.