mk4 rear calipers on Mk2 GTI

[mykdun]

OK, I thought I'd bleed the brakes today and after bleeding each caliper twice the brake pedal still goes all the way to the floor. I'm wondering if the original master cylinder is pushing enough fluid, I'd previously upgraded the front brakes to MK3 Golf (256mm?) and now with the MK4 calipers on the rear maybe I'm asking a bit much of the original master cylinder. I know a popular upgrade is to replace the original with a 22mm master cylinder from a later model, does the extra 2mm really make such a big difference?

[h100vw]

OK, I thought I'd bleed the brakes today and after bleeding each caliper twice the brake pedal still goes all the way to the floor. I'm wondering if the original master cylinder is pushing enough fluid, I'd previously upgraded the front brakes to MK3 Golf (256mm?) and now with the MK4 calipers on the rear maybe I'm asking a bit much of the original master cylinder. I know a popular upgrade is to replace the original with a 22mm master cylinder from a later model, does the extra 2mm really make such a big difference?

I reckon you've got air in the system somewhere. Or the master cylinder seals have flipped inside out. Try bleeding the master cylinder. If it just has 2 outlets, crack the brake lines and press the pedal, nip the line back up with the pedal at the bottom.

Some aftermarket master cylinders have 4 outlets and have 2 blocked off. You should be able to buy bleed nipples to go in the blanks, making the job easier. M10 x 1mm IIRC.

Also, when doing the back brakes you should set the rear bias valve, as though the car was fully loaded, this allows the maximum amount of fluid to the rear.

Gavin

[roccodingo]

I know a popular upgrade is to replace the original with a 22mm master cylinder from a later model, does the extra 2mm really make such a big difference?

YES, you need the extra volume for the larger caliper cylinder volumes.