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

Hi Christoph, here in UK I managed to source a refurbished "bomb" from a Citroen Hydraulics specialist company. I paid less than a hundred UK Pounds (180 or so US Dollars) including delivery.

I don't know if Citroen vehicles are prevalent in your Country, or if any similar company to the above exists. You may wish to make similar searches in South Africa.

Regarding differentiating a good from a bad bomb if you do tour the scrapyards would be along these lines....

1.Maybe..... Failure of a bomb would be as a result of loss of pressure/gas on the nitrogen side of the rubber diaphragm. In which case, would the hydraulic oil capacity be higher for a defective than a good bomb ? I seem to remember my fluid level diminishing as the bomb died. Sorry I don't have the capacity of a good bomb to hand .....

1. A refurbished bomb is likely to have been repainted ? ( mine was )

regards Ian

Hi Ian,

Thanks for your reply,

I just sent them an email, let's see what they say.

Citroen is not very common here (unlike Audi). Power steering pumps etc are no problem to get refurbished, but the break bomb seems to be. I got the impression, almost no-one cares, if it's gone. The Audi dealer doesn't have one in stock and they have stock of most items which fail regularly. I went to a specialist break and clutch place some time ago because of my clutch and asked the owner to check out my breaks. I told him that I thought they were not as good as the ones from the other Audi I have. He drove the car and said something along the lines: "You can get the wheels to lock, that's pretty good". Here, some consider their breaks to have a problem if you press hard and the car doesn't stop.

When I figured out the problem with the bomb myself (after reading about it on the internet) I went back and he confirmed it immediately when he pressed the pedal. But even with broken bomb, the breaks perform better than most other cars he gets in.

What I am getting at is: I assume that on 80 to 90 % of the cars the bomb is not replaced when broken, unless it is serviced at the dealer, in which case there is probably a slightly higher chance. But they will not use a third-party refurbished one, so there is no marked for those here.

The problem is that the fluid is gone by the time the car is scrapped. I was wondering whether it is possible to look into it or stick something in to test how big the space in the bomb is. But I don't know what they look like inside.

If anything, the ones on the scrap yard will be official audi refurbished ones. I don't know whether they repaint it.

Thanks for your help anyway. I guess I just have to go to a big scrap yard and try to see a few.

Christoph

Christoph,

DO NOT tamper with the bomb! It is not called a bomb because it looks like one, it's called a bomb beacause it IS one. Before removing it you must make sure it is discharged by pumping the brake pedal several times with the engine off until the pedal goes hard. If you don't, you will find out why it is called a bomb as you look at the stump that used to be your hand! I would advise you not to try to take it apart, or poke anything into it for the same reason.

Hi Jim,

Thanks a lot for your warnings. I will defenitely keep them in mind.

However, I think I didn't make clear what I intend to do. I will try again:

- I am assuming that more than 50% of the old Audis in South Africa drive around with a broken bomb (as explained in detail in my earlier message).

- If this is the case, more than 50% of the bombs on the scrap yards will be bad as well.

- When I go to a scrap yard to get one, I would obviously like to get a good one first time round. They refund the money if I return it, but I still don't like the idea to get one, to put it in, to take it out again and to bring it back.

==> I need some way to destinguish between a good and a bad one on the scrap yard. And that is where my questions started.

I thought of two ways:

1.) If there is anything which tells me whether it is the original one or whether it is a factory-refurbished one, I have a starting point. The latter has obviously a much higher chance to be a good one than the first one.

2.) If there is anything which I can see, test or whatsoever on a scrap bomb, it would help. That's where the "poke into it" came from. My idea was that if the bomb lost all of it's gas, the devide between fluid and gas might not have moved back in the end. My idea was:

- take a scrap one

- look into it or stick something (a small wooden stick for example) into it just to see whether this deviding part is further back than on a good one. I obviously don't want to leave the stick in there. Carefully sticking a small wooden stick into a bomb which wasn't used for months will certainly not make it explode.

So if anyone has an idea on how to perform a test on a scrap bomb, please let me know.

In the mean time I did some more research on 1.) above:

I have two Audis, one 1989 and one 1988. Both are type44 and both are called "500" here. I believe the older one corresponds to the 5k and the "newer" one to the 100. Anyway, the bomb works fine on the older one and not at all on the newer one (which sort of confrims my 50% assumption, just with a very small sample). I lifted them both up a bit and climbed underneeth to see whether I could find any differences. And I did:

Model No: good: 443 612 061 H bad: 443 612 061 B

--> I don't think the H and B has much significance, but I might be wrong

No under the model no: good: 032-4317-182-541/75 bad: 032-4317-052-541/80 I guess these are seriel numbers and don't think the differences are siginifcant.

Extra letters on the round part oposite the pipes: good: "FS" bad: "A4" no idea, what they mean.

Additional number: Now, this could very well be significant: good: A bit to the right of what I think is the serial number, the good one has "087/EE" It is not engraved as deep as the serial number, it must have been done with a different machine. bad: There is nothing like this.

My thoughts: The good one is refurbished, and the additional number was added when it was refurbished. If this is true, it would help to look on the scrap yard for one whith similar additional numbers.

However, a test as in 2.) above would still help, since even a refurbished one might have gone past its life time.

Thanks for any help,

Christoph

P.S. I have taken a few photos of those numbers, if anyone is interested to put them on a web page, please let me know.

... and once you've fixed the faulty one, your small sample will suggest that the bomb never fails!

The final letter is a VAG revision identifier, and suggests to me that the bomb in your old car is a newer version than that in your new car. This would mean that the bomb in one of the cars has been changed - almost certainly a newer part has been fitted to the older car.

Christoph, The "bomb" is such an easy thing to change, it takes about a grand total of about 20 minutes to do on my '87. No, there is nothing you can push into the thing that will tell you if the internal diaphragm is good - it appears that, over time, the rubber diaphragm becomes (or _is_) permeable to the nitrogen. Over time, it leaks out (either to the brake fluid, or the power steering fluid, or both) and you lose the assist when the engine is off. The procedure for making a non-rechargeable accumulator into a rechargeable one - one that, should you lose the charge, you can get filled at your local heavy equipment service station (pressure accumulators are used on backhoes/excavators/etc. - where loss of hyd. pressure in an engine stall would also be deadly) is somewhere on Audifans.com. With a discharged bomb (zero pumps of the pedal to total loss of brake assist) all it takes is a drill, a dremel tool, a magnet (to remove cuttings), the correct tap, and a high pressure Schraeder fitting (get the one that holds over 3k psi). The one I have on my car was done by Russ Southerlin (I hear he's not doing them anymore) - it hasn't needed a charge since I got it a couple of years ago. All the usual disclaimers apply. Cheers! Steve Sears

1987 Audi 5kTQ - rechargeable accumulator installed and working at a fraction of the cost of a new one. 1980 Audi 5k - with the old school vacuum assisted brakes...oh, is that _new_? 1962 and '64 Auto Union DKW Junior deLuxes - brake assist = long brake pedal (SPAM Blocker NOTE: Remove SHOES to reply)

The bombs on my two UK Avants are slightly different.

[1] is an early 1988 (pre-revision) 100 CD Auto model with self-levelling suspension. [2] is a late 1988 (post-revision) 100 base model with std. suspension (and subject of my 'Brakes hissing' thread).

On [1] the bomb is part no. 857 612 061B, [2] is now stored elsewhere so can't check. [1] has an extra port for the suspension (not present on [2]) and the smaller end is a round shape, while it is a squarer shape on [2]. Apart from that the fittings appear to be in the same places although I suspect the banjo connections may be different - in the same way they are on the pump. i.e. latter models used o-rings, earlier models copper washes. You can use a new style pump with old style banjos but not vice-versa, so I suspect the same may apply to the bomb. [1] gives me about 15 assisted brake presses, [2] gives 0!

Gary

Hi Steve, Peter and Gary,

Thanks for all your advice.

Ok, it took me about an hour to get the car lifted up and onto the stable stands. Then it became dark, so I decided to continue tomorrow. Ok, without my two little helpers (2 and 5 years old), I would have probably done it in half the time.

Thanks, that's interesting. Only last week I thought "If somehow I could put a valve in there and pump the stupid thing up again ..." But I thought it was just one of my crazy ideas and dismissed it. I didn't even consider to ask here.

I found a few old threads on audifanc (late 1997 and early 1998), espectially this one (which seems to be the original procedure):

Is that still the procedure, or are there any refinements, which were developed till then? I tried to search, but couldn't find any.

Anyway, I am not there yet. What I did today:

I went to a scrap yard, but they had only one and that looked like my older one (no extra numbers). I took it since they only charged ~$20 (but canot return it). I will now try to exchange it tomorrow. If it works (which I doubt), fine. Otherwise, I will consider the recharge or I might go to a bigger scrap yard and have another try to find one. Will see.

Of course, with this one you have to be a bit more careful than with the odometer advice (btw, I also got an old instrument cluster. They sold it for ~$15, since it was marked as "doesn't work, spares only". I took it apart with my two little helpers and there you are: The little gear is still ok. I am wondering what condition that Audi must have been in, with only 101k-km on the clock after 15 years. An old mechanic pointed that scrap yard out, which is a bit on the country side (as opposed to the Cape-Town places, which are as far away from here). He said, that on the countrysite they take better care of their cars and go longer distances less often. The engine I once got from that place is as new ;-).

But back to the bomb: Will let you know whether it's ok or not.

Thanks again,

Christoph (who should have gone to bed earlier, I might be so tired that I forget to pump the peddal before the job).

What is a "brake bomb"? is it a Wheel cylinder?

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)

Thanks Steve! Excellent explanation.

Hi again,

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 ...