adjustment on the master cylinder to adjust the pressure for the back brake calipers

There are no adjustments to the master cylinder, plus the rust issue would not be a master cylinder or pressure issue. That is caused by the vehicle sitting for long periods of time.

When a vehicle sits for long periods of time, it does not have the rust knocked off to keep them from pitting.

Here are a couple guides that will help with replacing the rear braking system and adjusting the parking brake:

https://www.2carpros.com/articles/how-to-replace-rear-brake-pads-and-rotors

https://www.2carpros.com/articles/parking-brake-shoe-replacement

I would suggest making sure this vehicle gets driven at least once per week and put it through some pretty good braking to make sure the rust is knocked off the rotors.

Please see below for more info on this.

Thanks