Rear disc brake swap 72 K20

[Haybag]

Looking for some help. Swapped a 14 bolt from a 2002 Silverado into my 72 K20.Issue I am having is not getting enough fluid volume to operate the factory 02 dual piston calipers. Currently have the stock 72 master,booster and a disc/ disc prop valve . The 72 booster won't accept a larger bore master cylinder,any thoughts?

[Richard]

I think there should be no problem with the stock master. Would make sure the rears are fully bled. Once bled the dual piston caliper does not really use much more fluid than a single piston. You are just pressurizing more fluid. Is the proportioning valve adjustable?

[Haybag]

Proportion valve is for a disc/disc, but not adjustable.