There is no adjustment on the factory prop valve... and front discs should typically get full pressure through a simple "T". The rear drums need the adjustable prop valve typically to balance F/R braking.

Manual discs are challenging... if the piston area in your front calipers is smaller than OE, which I guess it is with the SSBC setup, getting manual to work well is really hard. A smaller bore master, eg 15/16" may help though you need to know what you are starting with.

Was your car manual brakes to start with? The reason it matters is he pedal setup gives more leverage with manual brake setup.