Ian Borg

So,I have a w203 c200 cdi, and lately I started feeling my brake pedal going low when stuck in traffic. I told my mechanic about it and he said its the master cylinder.
so he changed it for me, but still I have the same problem, when I leave my foot on the pedal for a few seconds it starts sinking slowly slowly.
my mechanic attached the diagnostics tool but got no errors.
even he is baffled at what it might be because he said it was a classic symptom of a failing master cylinder.
any ideas as to what it might be?

ItalianJoe1

I've seen an internally bypassed abs module do the same thing, looked just like a bad master cyl. Only way we confirmed it was cycling the exhaust valves closed via a scanner until i found the pedal stable with one commanded closed, meaning the inlet was leaking at times when it was supposed to be closed.

Ensure there are NO leaks anywhere else, the calipers all move freely (I've seen stuck pistons feel similar as well). I feel like the ABS module is a last resort fix, they very rarely fail.

Ian Borg

ok so the best thing is to tell my mechanic to bleed the system again I guess.
could it be an abs sensor maybe or anything similar?
the thing is the brake pedal feels exactly the same as before the master cylinder was changed. Maybe the MC is faulty? I'll tell him to check calipers and pistons too, better start from there I guess