usually with these symptoms its the master cylinder but ive seen a few times where the abs module is the culprit, my 90 fullsize had a issue where the brakes would work but the pedal would bleed down when you were at a light so basically it would go down like 2 more inches while you were stopped, i thought it was the master, but ended up being that stupid rear abs module

 

Tried a reverse bleeder and found a small leak hidden up front replaced that but now my 4wd went out on the way home so now to fix that