Your car came w/a distribution block that contains a brake warning switch that illuminates a dash light if there's a pressure differential between the sides of the system that would indicate a leak. There is no "proportioning" or "metering" functions to this distribution block- it just routes the fluid to where it needs to go.
Up by the master cylinder (MC) there should be a "hold off" valve that "that mounts on the stud of the MC on all factory 1967-70 disc brake cars. It is used in conjunction with the distribution block mounted on the frame, which is the same on factory disc and drum cars. This valve holds off the front brakes to give time for the rears to activate. At a calibrated pressure, the valve allows fluid to pass and engages the front brakes. This valve is the same on all GM cars, but the bracket is different for applications...".
It should look like the one shown below:
If this valve is malfunctioning, the brakes can drag.
I would also say to be sure
that you've eliminated the possibility of the front rubber brake lines being pinched by the brackets that hold the lines from fouling the suspension. These brackets are usually mounted on the lower control arm, and can rust up in their centers, causing a pinched hose over time, or can be damaged.
The hoses themselves can become delaminated in their ID, causing the pressure to the calipers to either not apply correctly or not release correctly.