I have a 2006 Chevy express van 1500, I replaced the master cylinder with a re-manned because the pedal was mushy, it was good for a few days, same issue. Replaced it a second time good for about a week but also with another re-manned. The pedal does not get hard, breaks have been bench bled could it be the hydro boost? Not sure what else to look for, when I replaced cylinder it was good for a while with a decent pedal and now back to that same mushy pedal that goes to the floor.