Last question first, no replacing the rears with disks will not improve braking, but will look nicer. I replaced my master cylinder with the servo and cylinder from the punto(it is not a bolt on and does create a few problems,mostly clearance), pedal feel is a lot better as well as stopping power(wheels do lock up really fast at about half pedal). My rear hubs are off a punto turbo so it does have disks,i had problems getting the calipers on the back to work right, they would lock up before the front and i had to replace the valve that controls the ratio with the one off the punto turbo. My fronts are still on the drawing board but will probably be the punto stuff i already have.
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If you have any questions don´t be afraid, ask away. I´ll be glad to help.
If i´m online you can try my site, it´s mostly storage http://www.smokeme.ipupdater.com/everything