Master Cyl: 89-92 W/Perf Braking (New)IF your Trans Am or Camaro is one of those fitted with the special performance PBR 4 piston calipers.. different disc rotors to others etc.. Bore Diameter (N) 24mm Primary Port Size M11 x 1. 5 Secondary Port Size M12 x 1. 0 Then this is the master cylinder you must use only for 89 92 Trans Am & IROC.. particualarly but not limited to the very top of the line Camaro or GTAs. These were originally made in Australia by Girlock under strick GM License to