I just found the following message on google while looking for hints
on brake fluid bleeding. It's two months old now, but it still cought
my attention (copied from google, my reply is below):
I am just guessing here, but could it be that the brake bomb is gone?
On my Audi 100, 1989 it took me quite a while before I realised that
this is the problem. In most posts about the brake bomb, they mention
that it's main purpose is to give brake assistance when the engine is
off. However, my impression is that the most important part is to have
a reservoir so that the brake assist works instantly. Before I
replaced my brake bomb, the brake worked normally when pressing it
slowly. Only when pressing it suddenly, there was a short
(~half-second) delay before the pump could build up the pressure and
then the brake paddle went down further. The above sounds very much
Indicators whether this could be the problem are:
- Engine running: When the brake pedal is prest suddenly, it feels
hard first and then goes down further.
- While driving: When suddenly braking, the brakes are at first weaker
and then increase in strength as the pedal moves down.
- Just after the engine is switched off, the brake pedal is hard to
Working bomb: Just after the engine is switched off, the brakes still
work fine. After pressing the pedal down 20 to 70 times, it suddenly
For more details on the brake bomb and how it works do a google serch.
I agree. I've driven cars with both a bad "bomb" (535i) and failing
master cylinders (various) and the OP's issue sounds like it's in the
MC, not the bomb. replace the MC first then look at the bomb if it's
still not quite right.
replace "fly" with "com" to reply.
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