Replacement o-ring for brake bomb (part NLA)

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thrty8street
Posts: 577
Joined: Feb 21, 2011 1:13 AM
Location: Maryland

Replacement o-ring for brake bomb (part NLA)

Post by thrty8street »

Does anyone know where I can source a replacement o-ring for the brake bomb? My car apparently did not have one, which is probably why it was loose and leaking.

It's #5 in the diagram (pt.# 34331119868) which is NLA. The size is 15.3x2.4mm.
https://www.realoem.com/bmw/enUS/showpa ... Id=34_0276

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https://www.fcpeuro.com/products/bmw-o- ... 4331119868
https://www.autohausaz.com/pn/34331119868


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Thanks!
a2058
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Location: New York, USA

Re: Replacement o-ring for brake bomb (part NLA)

Post by a2058 »

I bought a bunch here:
https://www.oringsandmore.com/metric-bu ... um-25-pcs/

They also have ones in Viton, but I figured the original are black rubber/nitrile so should be ok with these black ones... lets see...

Just bought an ATE brake accumulator just in case as a backup recently and I thought it didnt come with an O ring but it was actually in the box, buried behind some things in there... so anyway, it looks to be the similar black O ring I got from this other source!
thrty8street
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Joined: Feb 21, 2011 1:13 AM
Location: Maryland

Re: Replacement o-ring for brake bomb (part NLA)

Post by thrty8street »

Thanks! Just ordered some. I'm also getting a chain clamp tool to tighten that thing down.

https://www.amazon.com/TEKTON-3960-Lock ... C56&sr=8-5

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gadget73
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Re: Replacement o-ring for brake bomb (part NLA)

Post by gadget73 »

McMaster-Carr will sell you a bag of 100 for a few bucks.


The old ATE system on my Lincoln runs a similar (same?) accumulator. The OE ones have a socket key welded to the top, the replacements do not. I used a cloth strap wrench on it. Wasn't quite brave enough to try a chain.

Just out of curiosity, where did you get the ATE unit? The source I used to know of has gone out of business, not sure who has them anymore.
thrty8street
Posts: 577
Joined: Feb 21, 2011 1:13 AM
Location: Maryland

Re: Replacement o-ring for brake bomb (part NLA)

Post by thrty8street »

gadget73 wrote: Jul 29, 2022 8:42 AM McMaster-Carr will sell you a bag of 100 for a few bucks.


The old ATE system on my Lincoln runs a similar (same?) accumulator. The OE ones have a socket key welded to the top, the replacements do not. I used a cloth strap wrench on it. Wasn't quite brave enough to try a chain.

Just out of curiosity, where did you get the ATE unit? The source I used to know of has gone out of business, not sure who has them anymore.
I'll wrap the accumulator with a towel before wrapping the chain around it. I got it from AutohausAZ. Here is the model:

https://www.autohausaz.com/pn/34331162054
gadget73
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Re: Replacement o-ring for brake bomb (part NLA)

Post by gadget73 »

Thanks. I'll confirm which one I need and maybe grab one for a spare. The one on the car now is so-so but I can tell its not 100%. A good one gets 3-5 pedal applications before the pump kicks on, I'm getting 2. When the pump kicks on every time the pedal is pressed, its time.

The ATE rig that Ford used is pretty goofy, if the accumulator is dead you get no brake assist and no rear brake pressure until the pump catches up. Its not like a normal master cylinder and booster setup where if the ABS doesn't work you still have 4 wheel power brakes. I don't even have a master cylinder, its part of the ABS assembly, and the only thing it does is operate the front brakes. I can tell you from experience that stopping one with just the front brakes and no boost isn't real fun.
Blue Shadow
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Re: Replacement o-ring for brake bomb (part NLA)

Post by Blue Shadow »

Glad to see a replacement o-ring is available. I might need to find a few.

The loss of the accumulator is completely different between my 5 and 7. The 7 gives a nice no-brakes feeling on first application then a firm pedal as the pump catches up. The 5 just flashes the dash light and gives braking performance in most instances. If the engine rpm are down then there is little added oomph from the system and one worries if the car will stop, then the pump catches up. This is a rare happening compared to the 7ers lack of braking before being saved.

I don't see how the accumulator can wear out at Thain has experienced. Isn't it just a divided container with a membrane and high pressure gas on one side and the power fluid on the other?
a2058
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Location: New York, USA

Re: Replacement o-ring for brake bomb (part NLA)

Post by a2058 »

I got the ATE unit from Fcp euro, it was on sale last week for $114.92. My original BMW one failed as soon as i got the car back in 1997. I bought a Febi brand accumulator back then which lasted 7 years. Then I switched it to ATE and its been going strong ever since, of course I don't drive it as often nowadays. I felt these are probably soon to be NLA items, might as well have a backup now...
Mike W.
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Re: Replacement o-ring for brake bomb (part NLA)

Post by Mike W. »

thrty8street wrote: Jul 29, 2022 8:25 AM Thanks! Just ordered some. I'm also getting a chain clamp tool to tighten that thing down.

https://www.amazon.com/TEKTON-3960-Lock ... C56&sr=8-5

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I wouldn't. Generally O rings seal on the edges, on the sides, not on the bottom, so torquing it tighter will not reduce leaking. It will reduce it unscrewing itself, but I haven't heard of that as an issue. I know it's tough to get it into your head that tightening it more will not make it seal better, but I wouldn't use a chain to tighten it.
RossDinan6
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Joined: Jun 24, 2006 11:23 AM

Re: Replacement o-ring for brake bomb (part NLA)

Post by RossDinan6 »

You are right about no need to over torque O-ring fittings, though most use the "if tight is good tighter is better" philosophy it seems. To clarify on O-rings sealing, they compress in their cavity when tightened, changing shape to fill the space designed for them. That's why an old heat cycled O-ring can have a square cross section when removed.
thrty8street
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Joined: Feb 21, 2011 1:13 AM
Location: Maryland

Re: Replacement o-ring for brake bomb (part NLA)

Post by thrty8street »

Would it be a bad idea to add some blue thread Locker on the accumulator threads? I want to avoid this thing coming loose again. Last time I just hand tightened it, albeit without the o-ring, and it came loose.
Blue Shadow
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Re: Replacement o-ring for brake bomb (part NLA)

Post by Blue Shadow »

They don't come loose. They are usually a pretty tight fit needing work to remove. Properly torque the unit on with the correct o-ring and don't worry about it until the bomb needs replacement again.
Mike W.
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Re: Replacement o-ring for brake bomb (part NLA)

Post by Mike W. »

thrty8street wrote: Jul 30, 2022 8:14 PM Would it be a bad idea to add some blue thread Locker on the accumulator threads? I want to avoid this thing coming loose again. Last time I just hand tightened it, albeit without the o-ring, and it came loose.
You will regret it if you ever have to change it again. And if you sell it and someone else has to, they will curse you mightily.
thrty8street
Posts: 577
Joined: Feb 21, 2011 1:13 AM
Location: Maryland

Re: Replacement o-ring for brake bomb (part NLA)

Post by thrty8street »

Thanks, everyone for your input! Here's a quick update - the o-rings arrived. Installed it on the accumulator and put everything back together. I did not use the chain clamp or thread locker, as recommended. I also installed brand new brake pressure switches to make sure there are no leaks anywhere. I will finish up tomorrow, replacing the fluids and bleeding the system. If it leaks ATF, I am going to pull a Steve-O on myself.

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gadget73
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Re: Replacement o-ring for brake bomb (part NLA)

Post by gadget73 »

Blue Shadow wrote: Jul 29, 2022 11:08 AM Glad to see a replacement o-ring is available. I might need to find a few.

The loss of the accumulator is completely different between my 5 and 7. The 7 gives a nice no-brakes feeling on first application then a firm pedal as the pump catches up. The 5 just flashes the dash light and gives braking performance in most instances. If the engine rpm are down then there is little added oomph from the system and one worries if the car will stop, then the pump catches up. This is a rare happening compared to the 7ers lack of braking before being saved.

I don't see how the accumulator can wear out at Thain has experienced. Isn't it just a divided container with a membrane and high pressure gas on one side and the power fluid on the other?
I think the membrane leaks or something. Maybe it gets very slightly porous over the years and the gas charge bleeds down? Not honestly sure but there is usually a gradual degradation in how they perform. Sometimes it just goes from OK to not OK though. But yes its just a steel ball with high pressure gas on one side and brake fluid on the other. Same basic thing as the Citroen hydropneumatic suspension accumulators.

does the 7 have that same ATE system my cars use?

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Lincoln remote mounts the reservoir but thats the unit.
Blue Shadow
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Location: SE PA

Re: Replacement o-ring for brake bomb (part NLA)

Post by Blue Shadow »

The 7 has the same system as the 5.

Shown above your post is the power flow regulator with the brake bomb attached. It receives fluid from the pump and supplies both the brake servo and the steering box. The brake servo is just a power supply unit and separate from the brake master cylinder which the servo operates through a piston. Some swap the hydraulic system for the vacuum system used in the 528e.
gadget73
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Re: Replacement o-ring for brake bomb (part NLA)

Post by gadget73 »

ah, so no electric pump like mine involves. You've got a version of hydroboost. The diesel runs that, but no ABS on that. I think that has some sort of accumulator build in to the unit, but its not the threaded ball like this. It works and I've never felt too much need to poke at it. If it ceases to work I've got a complete rebuilt spare unit on the shelf that has zero miles.
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