1995 Chevrolet Beretta replacing the Master brake cylinder

Sometimes they work? Can you explain this a little better? Do they hold if your foot is on the brake? Or does the pedal creep down, then you lose brake holding power? Its possible there is air in the system. Drum in the rear? Also, did you bench bleed the master before installation? What is the reasoning for changing the master? Have you checked for any fluid leaks? Any and all extra info you have will help us further diag this.

I have seen people top off the master with powersteering fluid by mistake, and cause havoc on all brake components. Easiest way to tell if fluid was mixed is by checking bladder underneath the master cylinder cap. If it is severely swollen, chances are the fluid is contaminated.