91 ranger soft brakes. master or RABS?

[bizzyizzy]

ok here is the low down.
91 ranger 4x4 4.o 5speed
new rear line from master to rear rubber line. previous owner no leaks no problems.
new lines from t to wheel cylinders. new everything in the rear drums. all adjusted properly.
rear brakes bled properly.

here is my problem. when the truck is off brakes build presure quickly as the should and do now see to lose pressure the drums engage and everything seems fine. so took it for a test drive and the brakes were spongy after startup the truck seems to nose dive more than it should and it was wet out so it was hard to tell if the brakes were locking up. after a couple hard brakes rabs light and red dummy brakelight came on. back in garage. next day lights off rebled brakes to make sure checked all lines no leaks. so here is the problem if the pedal is soft while running but holds pressure when the vehicle is off should i be leaning toward the rabs hydralic control or should i be thinking more a master cylinder. thanks in advance and sorry for the long read

 

[bizzyizzy]

clarification the brakes are soft and go to the floor not spongy per se