There is no proportioning valve in your car.
It had drum brakes.
Read through this one
1970 Chrysler Imperial Hydraulic Brake Service Guide from the Master Technicians Service Conference Session 274 or watch the filmstrip.
When you change to disks, then either add a proportioning valve or switch to a combination valve.
Hydraulic pressure does not act in direct relationship with drum braking system. The relative size of the cylinder does effect the force for a given amount of pressure but it leverages the shoes - and then on a servo drums (which is what we've got) the primary acts on the secondary increasing the force. Many original brake shoes had a different material for the primary and secondary shoes to fine tune the braking. And yes the relative size of the lining contact surface also plays a role because a given force will be spread out over a larger or smaller area. Lots going on in drum brake design.
Rear lock up. Be careful and there are no guarentees when the back gets loose. Fishtailing turns into tank slappers pretty fast. Catch a change in surface sideways and all bets are off.
