Spongy brakes are solved by installing residual valves.
I have a modern 15/16" MC on my '73 Dart on which I installed res.valves in both front and rear line (2 lbs valve in front / 10 lbs to rear).
This helped and firmed up brakepedal-feel.
But it also caused the fronts to grab the disc to hard at rest. Difficult to rotate the wheels when jacked up.
It cost me a full mpg in economy too.
Removed the front res.valve helped release thme again and get my lost mpg back.
But the calipers got a bit more spongy again.
Actually, the 73 style discs used a 15/16" bore POWER or a 1 1/32" bore MANUAL master cylinder.
This is bass-ackward from what seems logical.
Probably the brake pedal/MC strut is in a different hole which changed its ratio.
