93 K1500 Rear Brakes

I suddenly lost rear brakes, the pedal goes down further and the emergency brake light remains lit all the time. I have verified that the parking brakes are not engaged. I suspect the brake master cylinder, but am not sure about the ABS sensor and actuator. What is the best way to determine which of the components have failed? There is no fluid loss in the rear wheel cylinders.

you should pull the codes to find out why the lights on. make sure the rear brakes are properly adjusted if they are the problem might be the master cylinder(cheap), proportioning valve(cheap) or the abs unit(not cheap).

I had to put a proportioning valve on my 91 truck and new it was 191.00...yes for a small aluminum valve.
I went to the salvage yard and got one for 25.00 and it worked fine.
I had the same problem,i put 3 master cylinders on thinking it was faulty master cylinders but it was the valve,it will act just like a bad MC.