School me on Proportioning Valves 72

[Eturnit3]

I have a few minor issues and I need some schooling. The truck is a 72-k1500 disc front on d44 drum rear on 14b. Replaced everything, new cylinders new master cylinder, booster is functional but not new. Bled fluid to all corners, but on the test drive I still have fairly loose feeling brakes. I adjusted the read shoes out to just barely drag when I spin them by hand. Half way around the block, I get a brake light on the dash (the one that normally comes on when my e-brake is on) so I guess my biggest question is, what does the light mean. I assumed it meant no fluid or short in the switch thats on the proportioning valve. Can anyone explain what this switch/senser does? School me guys Im lost.