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.
Keep in mind that calipers merely relax, movement of a few thousands if not "knocked back" by a bent rotor.
The difference in fluid volume between the 2 diameters is minimum.
Once bled, the M/C just adds pressure to an already full system.
Cheers