Brake Bomb failure?

[DenverMDM]

So got the 8 all cleaned up and waxed from a long summer in the garage. Took it out and had a lack of getup on the highway. Brake pedal was mushy and could be pressed all the way to the floor with little stopping happening(scary at 75). Car started to shake like a tire was blown and I pulled over to see a bit of smoke from the passenger front wheel well. Got it home slowly. Driving back was like pushing the brake and accelerator at the same time. So from my research when this happened before albeit for 2-3 minutes not my whole trip, I thought it could be a failing brake accumulator. What do you guys think? Thanks in advance!

Maurice

[DenverMDM]

Sorry I put this in the wrong section, I blame my iPhone app!

[olinjohnston]

No, that would be a seized caliper since you had smoke and brake engagement on your way home.

Brake bomb failure only shows as a loss of brake boost pressure for a split second when hitting the brakes hard from high speed...or a "BRAKE ASSIST INACT." message on the MID upon startup after the car sits for a few days.

The accumulator sphere stores hydraulic boost pressure only. If it is faulty, the car would instead be more difficult to brake, not cause braking.

[DenverMDM]

Could it possibly be both? Cause I just did the brake accumolator test of pumping the brakes 20-30 times while the engine is off and although it got a little firmer it's still pretty spongy. I also think back on the time when I first noticed the problem and the pedal was mushy and I was in the mountains (yikes). Thanks for the reply!

[olinjohnston]

Could it possibly be both? Cause I just did the brake accumolator test of pumping the brakes 20-30 times while the engine is off and although it got a little firmer it's still pretty spongy. I also think back on the time when I first noticed the problem and the pedal was mushy and I was in the mountains (yikes). Thanks for the reply!

Yeah, chances are if your 1991 850i has its original accumulator sphere, it's due for replacement anyway. After 20 or so pedal presses, the pedal should feel like it's hitting a rock.

[DenverMDM]

Ok I'll put a new brake bomb on the next to do list. Looks like they've almost doubled in price since 2009. Would it be recommended to do a brake bleed at the same time? Which brand is recommended?

[m10n]

the bomb issue, is not related to symptoms you are experiencing which is far more serious than the preventative measure of replacing the bomb. you need to go through the brake system, and the hydraulic system , 2 separate issues. you need to rebuild/replace at minimum the faulty caliper, and bleed all fluid out of system with new fluid. check frictions and rotors, which may be warped now. then you can address the hydraulic system by replacing accumulator and all fluid to start with.

[smc850ci]

the bomb issue, is not related to symptoms you are experiencing which is far more serious than the preventative measure of replacing the bomb. you need to go through the brake system, and the hydraulic system , 2 separate issues. you need to rebuild/replace at minimum the faulty caliper, and bleed all fluid out of system with new fluid. check frictions and rotors, which may be warped now. then you can address the hydraulic system...

I agree.

If you're not getting pressure from the brake booster, then the accumulator will not have any thing to "accumulate".

You need to check your brake booster first, the one attached to the brake master cylinder. Check for leaks where they join, and inside the car where the brake pedal shaft goes through the firewall. If that's ok, then check the pressure regulator, the pump, and tank. Remember this is a two part system, brake fluid and Pentosin hydrolic fluid. They both have to work together to stop your car.

[mjrgroup]

No, that would be a seized caliper

+1

That would cause braking and subsequent overheating on one caliper. Fortunately, it's easy to rebuild calipers, search for it on YouTube.

While you didn't report symptoms for the following, it is also possible for a worn master cylinder to trap fluid on the wrong side in which case it would get harder and harder to brake with less pedal travel the longer you drive the car and operate the brakes. This condition would also apply pressure to all calipers when you are not pressing the pedal: fix is to replace master cylinder; something to think about when diagnosing. If you decide to replace your master cylinder, do the h31 hydraulic booster as well.

[DenverMDM]

H31 booster?

[mjrgroup]

Do a forum search for "brake booster"

The H31 brake booster is common to the early e32 and all e31 models. These cars use a hydraulic brake booster which is part of the power steering system (pentosin). The booster itself costs about $1200 at the dealer, so you want to rebuild yours. Fortunately, we have a resident member here named "Max Lumens" who sells rebuild kits. (http://bmw8.us/Brake%20Booster%20Rebuild.htm)

Check pics below for rebuild and function of the system:


ATE Brake Booster Diagram by MJR GROUP, on Flickr


ATE Brake Booster Diagram II by MJR GROUP, on Flickr


H31 ATE Brake Booster Internals by MJR GROUP, on Flickr


Core Brake System Rebuild (see description for details) by MJR GROUP, on Flickr


e31 Ate Brake Booster Rebuild by MJR GROUP, on Flickr


Old brake booster and master cylinder assembly by MJR GROUP, on Flickr







Brake master cylinder and booster unit by MJR GROUP, on Flickr

[ride850i]

Have to replace mine, I think I have the same problem.

[clockwork]

Yeah, chances are if your 1991 850i has its original accumulator sphere, it's due for replacement anyway. After 20 or so pedal presses, the pedal should feel like it's hitting a rock.

so I'm thinking my original brake bomb may be toast since it takes about 40-45 pedal presses till my brake feels like its rock solid. And my braking power sucks so I figure why not change it out with another since I HAVE another brake bomb.
BUT problem is, I can not loosen the two metal hose connections with a flare wrench or vice grips.
SO I have to ask... would it be ok to put a torch to the metal pipes or will it be too dangerous?

[Ahmed303]

I recommend you use PB Blaster or some other penetrating spray and let it soak for a while. Possibly overnight with some spray every couple of hours. Torch to the metal pipes may not work as the pipes will expand and make it even tighter. Not sure how much leverage you can get from the top but I did not have much of a problem going from the wheel well.

[clockwork]

Faisal, I'm going in from the back as you recommended and I've left WD-40 on it since I spoke with you. It seems to be fused together it seems I can see oxidization on the threads of the bolt. I've cleaned it all away with brake cleaner to the point of it looks like new metal and then I put WD-40 on many times over the last 36 hr period. dang. I figured for sure the vise grips would loosen the bolt, but it starts stripping around the sides.
I was going to heat the brake bomb, not the aluminum tubes... but wasn't sure how safe it may be.
Maybe I'm stuck buying two new hoses for the brake bomb too.

[Ahmed303]

Applying heat with a heat gun should be ok at the Brake bomb. You could also try a flare wrench and a loooong lever (a metal pipe of sort). That way you won't strip the sides.

[Bluesman]

I found using the recommended claw wrench 17MMs I believe allowed much more leverage because you connect a rachet to it and it grips nearly 75 & of the bolt. I had the same problem until I ordered the Claw set. I may be talking about the same wrench possibly because I am not familiar with "flare"

[Ahmed303]

I may be talking about the same wrench possibly because I am not familiar with "flare"

Yup Same thing



Or you can get a ratcheting one with a narrow handle to get a metal pipe in for extra leverage,

[clockwork]

first thing I tries was a 17mm flare wrench. I'd neve ruse anything but.
No success, just ended up sliding off the nut and wanting to strip it, and that was before I even added WD-40. now its even slipperier
I'll put some heat to the brake bomb for a few seconds. I don't wanna bust the diaphragm inside tho incase it IS still a good bomb/accumulator

[m10n]

You also don't want to ruin those pipes. I don't think you'll like the price on those. The bomb isn't cheap.

[clockwork]

M10n, I've cleaned them up, and tried with so much force, but the 17mm flare wrench just wants to slip off. I swear I'm openeing the flare wrench up as I twist its on that tight.
I'm trying to avoid using heat as much as possible.
I checked and the price for the 2 metal hoses are $52 each in Calgary. Not bad if it makes the replacement easier, BUT now I'm thinking why mess with something that may not be broke? I can't tell for sure if the brake bomb is kaput on the inside, so till Its an obvious problem, I'll let sleeping dogs lye. I have another brake bomb just waiting to be installed incase the one in the car ever does fail. I guess a cheap and easy part replacement/fix/test was not so easy after all...

[Ahmed303]

Did you try turning the other way as well to see if that will loosen it a bit?

[Bluesman]

Yup Same thing



Or you can get a ratcheting one with a narrow handle to get a metal pipe in for extra leverage,

Actually no there not. My Claw ones only have the head and you can attach a rachet with or without extension so you are above the bolt for leverage I will take pictures tonite when I am home from work.

[duje]

The pipes are not too expensive, but the problem is they are straight, and you need to bend them using the pattern from old lines. Not an easy job without proper tools.

[m10n]

Bluesman : sounds like you're talking about a Crow's Foot
Clockwork: i think you should be concerned about the symptoms, not the bomb. the bomb from your description is working, if you can press the pedal that many times its storing pressure, that's not the issue you were having, you were having a problem with the braking circuit, either a stuck caliper, stuck master, or something in the booster. i would change focus for now and figure out the problem.