Back brakes don't work.

[hotroddin78]

My 78 GMC's back brake quit working two weeks ago after i did a long burnout. they worked right after i did it but the next day they didn't. my brake pedal goes down 4 inches no problem and i dont even begin to slow down. also my brake warning light is on. I checked the brakes on sat and they were fine, no scores on the drums and plenty of pad. The passenger side was caked in grease( time to change axle seals) but i cleaned it all up. I Was thinking my proportioning valve. Any other thoughts on what might be up?

[BigRedChvy383]

sounds like the proportioning valve took a crap on you, mine is going bad i think or it hasnt been replaced when i needed it done, dad told me i just dont remember, i just havnt got around to really fixing it. i think they are like 5 or 10 bucks i could be wrong.

[forsaken_one]

Were you power-braking at the time?