GTI-6 Servo and Master Cylinder

[C.A.R.]

Definitely change the MC. I'm pretty sure the majority of 306s had the larger MC after about X-reg, but if your car is older than this it's highly likely it still has the smaller size on it.

I posted this up a while back. Should be stickied the amount of times I've had to copy and paste it now...

I don't claim any of the below to be 100% accurate, but it's as much as I know!
MC Diameter =
19.0mm - Fitted to very early 1.4, 1.6 and 1.9D cars (probably phase 1's) without ABS
20.6mm - Fitted to early 1.4, 1.6, 1.8, 2.0 and 1.9TD cars with or without ABS
22.2mm - Fitted to virtually all cars and engines it would seem after the phase 2 (16v 1.8's/2.0's) came about, most also had ABS - this is probably the one you have!
23.8mm - Fitted to all GTi6's and then from what seems to be the late phase 3's (2000-on) no-matter what size engine was fitted (this is what late HDi's / XSi's have installed) - this is the largest MC and the one which you want!