1992 Ford F-250 7.3 sinking brake when running.

I have a strange sinking pedal problem only when running and holding the pedal.(At stop lights.) Rear ABS light is on and was on before. the old Master showed that it was weeping at the booster side. Replaced. Vacuum pump 25-30, all lines hold vacuum. I couldn’t vac down the booster.

It did this with the old and new master, pedal is firm with the truck off. I have bled the system with a two man system and vacuum bled it, at passenger wheel cylinder/then drivers, Passenger caliper, RABS, and drivers caliper.(in that order)

NO EXTERNAL LEAKS. Rear brakes are new and adjusted. What the Heck? is there a way to check if the RABS if it’s bad?