brake "bomb": how to pick one at the scrap yard?

Dennis,
The "Bomb" is a nitrogen-charged hydraulic pressure accumulator that stores
pressure generated by the power steering pump to assist the power brakes.
The reason is is called a "bomb" is because it appears like the bombs in
cartoons - you see Boris and Natasha throwing one at Bullwinkle and Rocky
every now and then. Audi started using pressure accumulators in the
ur-Quattros, and then in the Type 44's (100/200/5000/V8) from around '84 to
the early 90's, when they reverted to the (less problematic IMO) vacuum
assisted brake system. From what I'm told, BMW also used pressure
accumulators for a time. Accumulators are most commonly found on large
hydraulic machinery - like excavators and bulldozers - to store pressure
assistance should the engine die (imagine an excavator stalling with the
bucket full of soil suspended in mid air.....the accumulator provides
assistance to lower the bucket-full to the ground). Those ones have
standard fittings so you can recharge them when the nitrogen leaks out = the
reason that most all of the Audi accumulators eventually go bad and need to
be replaced = $$$.
Cheers!
Steve Sears
1987 Audi 5kTQ - equipped wit da bomb!
1980 Audi 5k - no bomb
1962 and '64 Auto Union DKW Junior deLuxes - no brake assist (= light car +
big drum brakes)
(SPAM Blocker NOTE: Remove SHOES to reply)


"Dennis W" > wrote in message
m...
> What is a "brake bomb"? is it a Wheel cylinder?
>
>

Thanks Steve! Excellent explanation.

"Steve Sears" > wrote in message
...
> Dennis,
> The "Bomb" is a nitrogen-charged hydraulic pressure accumulator that
stores
> pressure generated by the power steering pump to assist the power brakes.
> The reason is is called a "bomb" is because it appears like the bombs in
> cartoons - you see Boris and Natasha throwing one at Bullwinkle and Rocky
> every now and then. Audi started using pressure accumulators in the
> ur-Quattros, and then in the Type 44's (100/200/5000/V8) from around '84
to
> the early 90's, when they reverted to the (less problematic IMO) vacuum
> assisted brake system. From what I'm told, BMW also used pressure
> accumulators for a time. Accumulators are most commonly found on large
> hydraulic machinery - like excavators and bulldozers - to store pressure
> assistance should the engine die (imagine an excavator stalling with the
> bucket full of soil suspended in mid air.....the accumulator provides
> assistance to lower the bucket-full to the ground). Those ones have
> standard fittings so you can recharge them when the nitrogen leaks out =
the
> reason that most all of the Audi accumulators eventually go bad and need
to
> be replaced = $$$.
> Cheers!
> Steve Sears
> 1987 Audi 5kTQ - equipped wit da bomb!
> 1980 Audi 5k - no bomb
> 1962 and '64 Auto Union DKW Junior deLuxes - no brake assist (= light car
+
> big drum brakes)
> (SPAM Blocker NOTE: Remove SHOES to reply)
>
>
> "Dennis W" > wrote in message
> m...
> > What is a "brake bomb"? is it a Wheel cylinder?
> >
> >
>
>

Hi again,

> The "Bomb" is a nitrogen-charged hydraulic pressure accumulator that stores
> pressure generated by the power steering pump to assist the power brakes.

I saw it mentioned in a few different places (like SJM etc.) but never
worried too much about it. What got me confused is that they generally
say it's there to store the pressure so that the brake assist works if
the engine is not running. Even when I realised that one of my two
Audis doesn't brake as well as the other, I didn't suspect the bomb
straight away.

I would assume the main purpose of it is to store pressure for sudden
emergency breaking. The power steering pump provides the pressure for
the break assist, but it does pump at a slow volume per time. When I
press the break down suddenly while standing (but with engine on),
there is a delay of about half a second before I can feel the assist
kick in and the pedal moves down further. And while it then moves
down, it doesn't do it uniformly, it feels a bit like an ABS, but not
as distinct. I assume the pump just doesn't pump very uniformly.

When I break while driving, I can get good break action and lock the
wheels without problems, just with a bit of a delay. Under normal
circumstances, it doesn't even feel as if there is something wrong. I
guess it's a bit like the seat belts. You almost never need them, but
in that one instance they might make all the difference. I am just
worried that this half second delay might one day just be the half
second too much ...

What makes it worse is the fact that 15 years ago, Audi didn't think
it was worth selling their cars with ABS in South Africa. I think it
took them until the mid-90s before they had it as standard in all
cars. Without ABS, you need to be able to control the breaks well. Now
imagin, you get a week action for the first half second and then it
suddenly starts to work.

But I actually wanted to give an update on my weekend experience:

I had closer look at the bomb I got from the scrap yard. Not only did
it look like an original one (which has very likely lost all pressure
as well), it has also a bend holder on one end. And when it inspected
it further and turned it up and down, some dark oil started to drop
out. It was so dirty that it was impossible for me to tell what colour
it might have been long ago. On top of that, there were lots of fine
metal pieces in there. I decided not to contaminate the rest of my
system and put it away. I will try to have another search on the
bigger scrap yards next weekend. If I ever decide to have a go on the
recharge, I will use the dirty one for a first practice run.

That's all for now,

Christoph

P.S. I sent Schrader Valves UK an email on the weekend to ask for the
South African distributor, but didn't get a reply yet ...
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