Proportion valve

Dual master cylinders were introduced in 1967,as per federal requirments.With a dual master cylinder,the front reservoir and piston served the rear brakes,while the rear half of the master cylinder operated the front brakes.On drum brake
applications,both reservoirs were of comparable size,but disc brake master cylinders used a larger reservoir to feed the front brakes.Since brake calipers cannot be adjusted to compensate for brake lining wear as the brake pads wear,the caliper pistons must protrude further and further from their bores in order for the pads to maintain contact with the rotors.As the pistons move from their bores,the cavity behind them grows larger and larger.Additional brake fluid is needed to fill the voids left by the pistons.Without the extra capacity of the disc brake master cylinder reservoir,the possibility exists that a smaller reservoir could run out of brake fluid.Dont gamble with safety.When retrofitting to disc brakes,always install a disc brake master cylinder.Additionally both ports on drum brake master cylinders and the front port on a disc brake mc used in to the mid 70's were fitted with residual pressure check valves.Residual pressure check valves were used to maintain a small amount of hydraulic pressure in the wheel cylinders at all times so the air could not work its way past the piston cups and into the wheel cylinders.
By 75 or so,wheel cylinders were being fitted with small metal pie pan shapped expanders inside the cups.All wheel cylinders manufactured since that time and all rebuilding kits have included these expanders.If you use a mc intended for an application prior to the mid 70's,it should have a residual pressure check valve in its front port. If you use one intended for an application after roughly 1974, it probably will not have this check valve. If you think your car may still have the original wheel cylinders, it would be a good idea to change them or at lest rebuild them. Early disc brake quipped cars used an additional valve, as well. A valve in the brake line that fed the rear wheels was typically mounted to the inside of the left front frame rail a foot or two back from the firewall. Chrysler called it a proportioning valve. The proportioning valve limited the pressure applied to the rear brakes in a nattempt to reduce rear wheel lockup during hard braking. By 1970, some models were also fitted with a metering valve. it helf off pressure to the front brakes until the rear brake shoes began to cntact the drums. By 1973, everything was built into one valve, called a combination valve, but sill often referred to as a proportioning valve. If you wish to maintain a completely original apperance with your drum to disc brake conversion, you will need to install the valve(s) that the factory would have used on your particular car. If a totally original appearance is not of the utmost importance, a combination valve from a 1973 or later car of the same body style should result in acceptable brake proportioning, and often a simpler installaiton.

I hate typing !!!!!!!!!!

Anybody planning on doing modification's like this on there A-bodys,let me suggest a book for you.
Chrysler Performance Upgrades by Frank Adkins
This book has all the answers.Engine swaps,Transmissions,cooling systems,fuel,electrical,rear axles,driveshafts,suspension mods,brake systems,etc......
Hope this helps.