ABS system for 94 Z71 ft/bk locking up..

Hi, well I have a problem on my 94 1/2 ton ABS system. I have checked the codes, none, bleed the lines numerous times, they cant seem to get evened out. Normal driving and all the sudden the back for instance will lock up and just smoke!!!! But it does in on front, and back....I cant figure what the problem is??? Can you help me QUICK. and PLEASE!!!!!

I just had the same problem on my 92 chevy. I had to replace the master cylinder. Its cost around 30 bucks for the master cylinder at checker. When you change it out make sure you get all the air out of your lines.