i have a 96 1500 that does the same thing...i have gone thru 2 new masters and the problem is the same...you can drive the truck but the pedal is squishy. i have bleed them several times...no change.
the truck has no ABS (at least it doesnt have any wheel sensors or fuses or relays in the appropriated spots) BUT...it does have a solenoid looking proportional valve looking thing, directly under the actual proportioning valve (see below pict, top one is the suspect). the valve looks like it could be rear only ABS but again, i dont think so (the rears will lock up)
this valve could be bad i think...causing internal bleeding and squishy pedal...just no external leaks or air in the system.