Hi, a few days ago I replaced the front brake pads on my Octavia. Once reassembled I had very soft brakes and the pedal slowly sank under pressure. The car's done 191k miles. A mechanic friend said the the pad replacement procedure had probably forced fluid back past the master cylinder seals. I therefore put a new master cyliner on today. The brakes are still soft (OK new pads take a while to bed in) but with the engine on the brake pedal still sinks slowly. It doesn't sink with the engine off.

We've put new fluid in and bled the whole system very carefully and there are no leaks anywhere. On the road I can do an emergency stop so that the ABS kicks in. Could the sinking pedal be caused by the new master cylinder being defective or should I be looking for something else?

One other thing - while it's sinking it seems to go past a point in it travel where a distinct click can he heard and felt through the pedal.

Thanks, William

Its not the slave cylinder is it?

Not much help im afraid as thats a random guess

Slave cylinders seem dry so I'd guess not. Thanks for the suggestion.

I read somewhere that the ABS pump might also need to be bled. Would air in this pump cause that 'sinking feeling'?

Did you have bleed nippled on the new master cylinder?

If you pump the brake pedal does it go hard?

How many miles have you done on the pads?

If it doesnt sink while the engine is off then it cant be air in the system or a faulty master cylinder as it would do it all the time.

I'd clamp off the front brake lines at the flexi and see if the brake pedal still sinks.

Hi Lummox, yes the new master cylinder soes have bleed nipples - they were duly bled. The new pads have about 30 miles on them now. With the engine on, if you pump the pedal it does get a bit harder but not much. Also if I put my fut on the pedal with the engine off it's good and hard. With my foot still on the pedal I turn the engone one and the pedal sinks slowly straight away.

Why were you suggesting I clamp off the front brake lines?

Why were you suggesting I clamp off the front brake lines?

To determine if the fault is at the calipers or before this point.

Air in the ABS valve block?, will need bleeding with VAG Com or dealer tool.

Been reading this topic with interest, having exactly the same problem since replacing rear pads on my Treg 1.9 Octavia. Came to the conclusion master cylinder was to blame, due to back pressure caused by using the Laser winding tool.

So do I need to replace master cylinder? or was this problem sorted in a simpler way.

Thanks, Adrian.