My guess is that the bomb is not the problem.

More likely that the first poster is having the problem due to a failing master
cylinder.

Christoph Bollig wrote:
> Hi,
>
> 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):
>
>
>>From: - view profile
>>Date: Mon, Jun 13 2005 6:28 pm
>>Email:
>>Groups: alt.autos.audi, rec.autos.tech
>>
>>Hi,
>> I think there is air in my rear brake lines, but bleeding is a
>>rediculous pain. I know the proportioning valve on the '84 is not
>>adjustable, but is there a way to disable it? I know the first ones on
>>Jeeps there was a button that could be held. I just want a way to keep
>>normal pressure or lot's of pressure to the rears while bleeding.
>>Right now I can bleed once and then the rears get closed off.
>>
>>My main problem is that I have either a leak or air in my lines, but I
>>can't see a leak and don't seem to lose fluid. If I press my brake
>>pedal slowly it goes about 3/4 or 7/8 to the floor, but still feels
>>like a normal pedal. When it does this I get pressure to all 4 wheels,
>>but probably about half of what I'd call normal pressure. If I push
>>the pedal more rapidly or fast it will only go half way and feel like a
>>normal pedal with full pressure, but only the front wheels will have
>>pressure. I'm hoping it's just air in the rear lines and under lots of
>>pressure that compressed enough to divert the flow and close off the
>>rears. If it's something more serious then maybe there's a way I can
>>make the fronts always have the pressure? Right now I can brake fine
>>with the mostly depressed pedal, I'm just worried when I go to get the
>>car inspected they will say it's inappropriate that way.
>
>
> Hi Chris,
>
> 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
> like this.
>
> Indicators whether this could be the problem a
>
> - 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
> press down.
>
> 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
> becomes hard.
>
> For more details on the brake bomb and how it works do a google serch.
>
> Christoph