what is the purpose of the proportioning valve?

Proportioning valve is to limit the brake pressure in the rear brakes during a panic stop: http://www.freeasestudyguides.com/proportioning-valve-diagnosis.html

In other word yes, it would be a good idea to have it, or your rear breaks will be locking up all the time. Brake pedal low? 2 things to check. The rear breaks need to be adjusted up to the drums so as they drag a bit, so the wheel is not free wheeling when you spin it. Then did you bench bleed the master cylinder before putting it on?

In other words yes, it would be a good idea to have it, or your rear wheels will be locking up on harder breaking. Break pedal low? 2 things to check, the rear break shoes need to be adjusted up to the drums so as the wheel is not free wheeling when you spin it, they should drag a bit. And then did you bench bleed the master cylinder before putting it on?

OK I understand this. Is the lack of the proportioning valve keeping me from building pressure in the entire system? I have bleed the system and have fluid in all the lines and the wheel cylinders but cannot get any pressure built up. Pumping the pedal does not result in fluid discharge from the bleeders. I am a seasoned amateur mechanic and I am stumped.

Did you bench bleed the master cylinder before putting it on? Also the rear break shoes need to be adjusted up to the drums so they drag a bit.

Yeah I did bleed the master. The rear shoes do need adjusting. I'll try this and install the proportioning valve. Maybe this will help. Anyway thanks for your interest and help.

Still messing with this problem. I have discs on the front and the original '52 drums on the rear. I am trying to use the master cyl. from a 2000 S-10. I do not have a proportioning valve installed.