Put a new NAPA master cylinder on my sons 94 dually and a new rear hose. It had the EGR SS hose on it so I don't think that was the problem but now, it will lock up the rears and kick the ABS in like it should and the ABS lights are out. The pedal is still not solid but it will stop now.
Also, got the B&D brake working again. A drop of moisture had gotten in the vacuum solenoid switch and hung it up. Took the switch off the truck, tore it apart, cleaned up the little control valve, and put it back together. The solenoid wouldn't click before but it does now. The brake works again. If your B&D quits working, start with the solenoid valve first and make sure it is free and stroking properly.