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Discussion Starter · #1 · (Edited)
My Mk4 GT PD130 ASZ appears to be leaking from the brake master cylinder - the brake fluid warning light on the info display flashed up when cornering - when checking the reservoir noticed that fluid appears to be leaking from the back of the master cylinder where it bolts on the brake servo (access and visibility is limited even with the EGR pipe removed !) - brakes appear to be working as normal with no warning flights etc since toping up - FYI I've owned the car from brand new and have regularly changed the brake fluid every 2 years - have covered 176k so can't complain.

Presumably I can assume the brake master cylinder will need replacing ? just seems strange that whilst the master cylinder is definitely loosing fluid, that the brake performance remains good ? can the fluid be leaking from any other point ? (clutch appears to working perfectly as well)

Going forward I've trawled through as many posts regarding replacing the master cylinder, Haynes brake bleeding procedure and have a pressure bleeder but do not have access to VCD/ VAGCOM - If air does enter the ABS system, is it an urban myth that the ABS can be purged by a controlled activation ? I'm near Bridgwater Somerset so there a number of specialist near me should I require a VAGCOM/ VCD ABS bleed - any thoughts ??
 

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I'd check both master cylinder bleed nipples are tight (don't overtighten them, just need to be snug) first but it could be the rear seal in the master cylinder.
I believe you can buy rebuild kits for them which would save you replacing the entire thing.

Bleeding the mk4 brakes is a different story. 99% of the time you'll end up with a floppy pedal which takes a combination of bleeding the calipers, bleeding the master, bleeding the ABS system and the stars aligning to get the pedal back to normal.
The VAGCOM ABS bleed proceedure has never really made a difference to me so depends where the problems lies when you have issues. You can pick up OBDeleven for around £100 which does pretty much the same thing just with a nice user interface.
 

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Discussion Starter · #3 ·
Turned out to be the primary side "top hat" seal (where the reservoir pushed in to the cylinder) was leaking - found this out once the master cylinder was removed - replaced with a £65 Febi Bilstein master cylinder ( seems well made for the price) - time consuming to fit, but if I had to do it again I'd say no need to remove EGR valve etc for access - I used a 13mm 1/2 inch deep socket and foot long extension bar to get to hidden nut and view with a small mirror - brake lines should screw by hand and only need a spanner to finally tighten to avoid cross threading - bled everything as per Haynes - master cylinder first (primary then secondary ) then front bleed nipples at same time then back at same time, then each bleed nipple in turn - No ABS problems once done and brakes working well - used a Gunsun pressure bleeder and a one one non return valve type bleeder. Didn't need to bleed clutch.
Reckon the top hat seal failed due to rocking/moving the reservoir when removing the intercooler pipe for EGR cleaning
 
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