C10 brake help needed...step inside

[bigblock73]

Ok, I have a problem with the brakes in my '75 C10.

History:
Last summer I was having an issue with the rear drum brakes being touchy and locking up when applying the brakes at times. In addition to that, I was also experiencing low pedal pressure, and somewhat inconsistent as well...meaning it took different pedal pressures to stop from time to time. With life getting in the way, I parked the truck and began working on our new home we purchased that summer.

Fast forward to July 2014. I took the rear apart, found a leaking drivers a side wheel cylinder, and passenger side axle seal. So, I went ahead and purchased new cylinders, shoes, hardware kit, had the drums turned, and also picked up new wheel bearings and seals. While everything was apart, I had the backing plates, drums and diff cover powder coated...figured might as well make everything back there brand new and not worry about anything for a while...wrong.

Everything has been reassembled, adjusted out and brakes bled, and to my surprise, I have worse pedal pressure and now my idiot brake light is on! I should add, it did not even touch the front brakes, or anything forward of the rear differential.

My inclination is that the master cylinder is bad too, what do you guys think?

What exactly triggers the idiot brake light to come on? Is there a reset for the proportioning valve. I presume the light is tied to the wires going to it. Could the valve be bad?

Looking for knowledgable advice here fellas, please let me know what the heck is going on, pretty frustrated at the moment.

Thanks!