resetting brake switch/valve

Hello all. The rear brake line on my 81 fairmont rusted out the other day. I replaced it, then bled both rear brakes. the pedal still didnt feel right so i decided to bleed the front brakes too. the master cylinder is split into 2 sections. 1 section holds fluid for the front brakes, the other section holds fluid for the rear brakes. While bleeding all 4 tires, fluid came out of the section for the rear brakes only. Throughout all 4 tires. No fluid at all came out of the side for the front tires (the bigger side). There is absolutely no air in the system, and the brake pedal still doesnt feel as good as it did last week.

Im assuming the valve/switch (pressure differential switch maybe?) thing underneath the master cylinder is the culprit, i just dont know how to "fix" it.
Can anyone help?

i wanted to add: the master cylinder was replaced last summer, the brake light on the dash has not illuminated once through any of this, and i have confirmed that all 4 tires do lock up when the brake pedal is pressed..