Upon renewing the master cylinder and bleeding the air from the system, the pedal really felt great, bitting really high as before tampering with the system. Next day it got worse but still driveable.

 

So, both new cylinders. It seems its drawing some air somewhere. All started after renewing the slave cylinder.

Not sure with this replacement parts are they of equal displacement as the original ones and does it really matter as if it is smaller maybe the piston is passing past the seals and some air is getting in the system, or some oil leaks somewhere and than on pedal release it sucks some air somewhere 

Did you fit a new seal where the pipe connects to the slave cylinder?

No, can be bought separately from the pipe?

Yes, item 28/28A here: https://skoda.7zap.com/en/cz/fabia/fab/2006-453/7/721-721025/

1K0721741 is a kit containing two o-rings and two 'more complicated' seals. On our 2005 car it was the second of these that was required, see images here

https://www.skoda-parts.com/spare-part/1k0798741-o-ring-seal-9-25x6-07x1-78-skoda-17391.html

If yours just requires the o-ring style seal, that is available separately as the second listed part number for item 28A, much cheaper https://www.skoda-parts.com/spare-part/1k0798741-o-ring-seal-9-25x6-07x1-78-de-14891.html

 

I can't completely see how this seal can be at fault, letting air in, but not fluid out, but I guess it's possible that an old, stiff seal could show strange behaviour.

Edited 7 November, 20205 yr by Wino

Yes it is the "more complicated" one.

I will try to source it locally.

No any oil leak is visible on the outside portion of the system, maybe some within the bellows.

I should dismantle the slave from the bell house and try to figure out if any fluid within.

Thanks.