My wife's 2010 Fabia hatchback 1.2 TSI brake pedal sinks when braking but if you pump the pedal it remains firm, mechanic could not find any leaks and the fluid remains stable, I am thinking this is a master cylinder issue but these are very expensive to just replace and hope for the fix, has anyone here had this issue and found the cause etc?  Also if requires a replacement master cylinder where is the best place to buy at a reasonable price ? (quoted around £300 from main dealers)

Hello Mike, welcome to the forum. Apologies in advance for asking the obvious question - has a full brake system bleed been tried?

From my understanding, bleeding modern brakes is not necessarily a simple job when attempting to clear the ABS module.   

Yes, the mechanic said the brake lines and system checked and fluid renewed with a pressure system, it passed the mot but if the pedal is depressed slowly it will travel to almost floor, but stops the car. I am just not happy to press twice when braking, one day it may fail..

It sounds to me as if the fluid is returning through the master cylinder and into the reservoir. Have you looked at getting a replacement seal kit for the master cylinder? Or a third party cylinder?

 

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Edited 6 December, 20232 yr by Jocko

With the engine off is the brake pedal hard high up and does not sink even with hard foot pressure ?

Then when you start the engine that sinking occurs ?  The pedal should sink a bit when the engine starts, that's normal due to the servo assist, but just an inch or so, not right down.

Asking as on my old Roomster I got air in the ABS unit and had to do an ABS Bleed to immediately fix that using VCDS. Normal brake bleeding does not purge the ABS.

My fault I got air in the ABS unit by running the reservoir dry while bleeding them. Only that VCDS ABS Bleed cured it.