Working on 98 Monte. Brought to me with very little brake pedal and very hard to stop. Checked brake fluid, fine. Checked for leaks, none. Pedal was fine with key off, but went to the floor with car started. Changed booster, no change. Changed master cylinder and ABS modulator(as a unit), no change. Bled the system properly. Any help in this matter would greatly be appreciated.

 

I have no idea, I'm having the same problem with my '99.