[AROWHED]

My 65 GP has a mopar aluminum 7/8" bore manual master cylinder, Dorman M39327. (No vacuum booster, engine only pulls 6-8" idling, and not considering any Hydroboost option).

The master rear port is plumbed to the front disc brakes, which are 1970 Bonneville 3" piston calipers and matching rotors.
The master front port is plumbed through an adjustable valve to 2016 Chevy Surburban rear disc, single 2" diam piston calipers, and 14" rotors. (20" rim, 30" tall rear tires).

Modified pedal for 7:1 ratio, but it does take a good amount of leg. Adjustable valve is set to allow full pressure to rear. There are also 2 lb residual valves in both front and rear lines, very near the master.

Problem is not enough stopping power. It will make the front end dive hard but cannot lock any of the tires on a hard dry surface. Stopping distance is definitely excessive. Bled it numerous times, yet I can still bottom out the master with a very hard push. That seems to indicate a need for 1" bore master, but will pedal effort climb even higher? Its already quite stout. Any other suggestions?