Front disc conversion on '65 Barracuda FS

yeah i'll second that....mine has 9 inch drums on the back (60s style duo servo self energizing) and single pot (slider) disc front.
without a proportioning valve the rear would lock up... new tyres, old tyres,bigger wheels nothing fixed it until i got a brake bias.

presume with the 4 pot front and 10s on the back things will be worse

issue is disk brakes need more fluid to move (look at the bore of the caliper pistons) and a higher line pressure to work properly than drums

so without the valve, by the time you get your front disks 1/4 to 1/2 ON the rears are already full on like you used both feet on the peddle

the valve lets you get the best out of the discs up front well before you have over powered the tiny wheel cylinders out back

even more necessary with 60s duo servo brakes at the back, these self power-on as the shoes ride round on the mountings.... they altered the rear brakes over the years as disc fronts become more commonplace to reduce this self energizing action.
duo servo was totally appropriate with drums all round .....not so with discs up front

look to the master cylinder bore size if when all said and done, it still feels like your brake pads and shoes are polished wood (no progressive braking and a confident stop only when you really are standing on that peddle)

a smaller bore would give longer peddle travel, greater subtlety in brake application and less foot effort...

Pressure in system = Force of foot/x sectional area of master cylinder
make x section area smaller and for the same force with your foot you get higher line pressure and maximum effort with the foot would produce a much higher pressure in the brake lines than before meaning a wider range of application from brake off to brakes fully on.

smaller bore cylinders at the back reduce the peddle travel but also reduce the force they apply to the shoes for a given pressure in the line, so benefits there as well if you can find some that fit.......
big wheel cylinder bore
Pressure in brake line x Area of cylinder x section=Force on shoe
smaller wheel cylinder bore
PxA(smaller)=F(smaller)

its a balancing act between what you can get for your brake set up and the fact that with modern tyres and modified car, things have moved well away from 1968 cross ply n drums set up.

these days you need to stop in a style closer to an ABS equipped Audi.... not a 1968 dodge dart...and you can, with the right set up if you can modulate that peddle and keep off the clutch, in the panic of an emergency stop.


Dave