15/16 is what I found after tons of research. I have the exact setup as you (180sx front/stock rears)
Thread is here:
http://www.zilvia.net/f/showthread.php?t=129909 (premie only forum - sorry guys) and my findings were here:
http://www.zilvia.net/f/showpost.php...3&postcount=26
"15/16" Master Cylinder
'86 Z31 Turbo MC part number is: 46010-18F20
'86 200SX MC part number: 46010-18F20
Autozone = Fenco M52060 $27 (w/ core)
Advance Auto Parts = Cardone 112060 ($37 w/core
Port locations on the 200sx and 240sx BMCs look identical (from pics) I'll be doing this upgrade when I finish re-doing my brakes. I'll try to remember to give an update on what happens."
It was recommended to me to NOT get the Z32 due to only having the 180sx fronts + stock rears. I hope I'm right...the brake bias is the same as stock 240sx. If anyone sees anything wrong with this, PLEASE let me know. I'll be buying this MC pretty soon and will update you guys.