MyE28.com

in the braking system of an E28 M5? I've searched here and Roadfly but am still not clear. Is it different than the "brake bomb". I can't tell from Realoem. I see the bomb, master cyl. , & regulator.

Does a "hydro booster" cost $800?

The "hydro boost" system (a GM term, AFAIK) consists of the P/S pump, regulator, "bomb" (accumulator), and brake booster. Of all of those, it'd be the regulator that lists for $800. It's all the same stuff as on the 535, BTW.

Thanks S.

Of those parts, I think it is the brake booster that I am trying to find information on. I don't see that part on any of the realoem diagrams under "brakes".

L_N_Love wrote:Thanks S.

Of those parts, I think it is the brake booster that I am trying to find information on. I don't see that part on any of the realoem diagrams under "brakes".

You are looking for the Hydraulic Brake Servo Unit. It's listed under Brakes>Brakes Master Cylinder

Rich

Thanks Rich. So its the master cylinder aka hydraulic booster. That clears it up. I was somehow wrong from my searchs in thinking that there might be another part in addition to the bomb and the master cylinder.

Glad to know that they are the same for 535s and M5s.

L_N_Love wrote:Thanks Rich. So its the master cylinder aka hydraulic booster. That clears it up. I was somehow wrong from my searchs in thinking that there might be another part in addition to the bomb and the master cylinder.

Glad to know that they are the same for 535s and M5s.

It's not the master cylinder, but the master cylinder does bolt to the booster. Go out and take a look at your master cylinder, you'll see what I'm talking about.

Rich

I see. I looked at Pelican Parts and see that there is a "Brake booster" listed. Expensive... > $500.

http://www.pelicanparts.com/bmw/catalog ... htm#item21

Steve Haygood or AutoHausAZ have better prices. I just purchased a new Brake Booster from AutoHaus, you won't find a better price (except for Steve). Trust me, I looked. Also, my local shop attempted (3) rebuilds between two different boosters, but they still leaked.

Cheers,

Chuck

While this seems to be the part that some have repaired by replacing an O-ring, I got mine from Steve.

The (2) different Brake Boosters that I had, could not be rebuilt. From my understanding, they have two seals, (1) on the brake pedal side and (1) on the brake master cylinder side. Most people replace the cylinder side, but the other side can leak also and no replacement seals exist for the pedal side (from my understanding). If you plan on keeping the car purchase a new unit.

Cheers,

Chuck

John in VA wrote:While this seems to be the part that some have repaired by replacing an O-ring, I got mine from Steve.

Do you know what pn or where the replacement o-ring came from? Mine just recently started leaking...

No BMW P/N for the o-ring, you have to source them from an aftermarket. ATE and BMW do not consider the Brake Booster a rebuildable item.

I know that others have been successful on rebuilds and have details in older posts. In my experience, rebuilding the Booster is a roll of the dice, may or may not work.

Cheers,

Chuck

How does the brake booster work though? I've always wondered. Also what is the difference between the the brake booster in the M20 powered machines and the ones found in the M30's. I know they are of different shapes but the 528e/525e does not have the brakes bomb, why?

Specifically how does engine vacuum multiply braking force which I assume means increasing the pressure on the hydrolyic fluid.

louielouie wrote:Do you know what pn or where the replacement o-ring came from? Mine just recently started leaking...

I don't know the size, but it's been mentioned here or on the Roadfly forum. Try a search.

bm0p700f wrote:Also what is the difference between the the brake booster in the M20 powered machines and the ones found in the M30's. I know they are of different shapes but the 528e/525e does not have the brakes bomb, why?

The M20 uses vacuum, the M30 uses hydraulics. The booster on the M20 stores vacuum (that's why there's a check valve), but the M30 booster does not and must rely on a separate hydraulic accumulator (the "bomb").

bm0p700f wrote:Specifically how does engine vacuum multiply braking force which I assume means increasing the pressure on the hydrolyic fluid.

Any pressure differential on an object will result in a force -- that's how all boosters work. In the case of vacuum-boosted brakes, the force of atmospheric air pushes a diaphragm into the vacuum-containing area. Also see:

http://auto.howstuffworks.com/auto-part ... -brake.htm

John in VA wrote:

louielouie wrote:Do you know what pn or where the replacement o-ring came from? Mine just recently started leaking...

I don't know the size, but it's been mentioned here or on the Roadfly forum. Try a search.

38x3mm

mine is dumping brake fluid on the garage floor.

If it's brake fluid, that's your brake master. If it's ATF it's the brake booster AFAIK.

Jeremy

Also, my local shop attempted (3) rebuilds between two different boosters, but they still leaked.

Maybe, but I'd be suspect about your local shop. I rebuilt mine, well, replaced the leaky "O" ring probably 3 years ago now, with no further leaks. There's about half a dozen seals and O rings in there that can leak, but 95% of the time it's the big one on the outside.

Jeremy wrote:If it's brake fluid, that's your brake master. If it's ATF it's the brake booster AFAIK.

Jeremy

Its brake fluid. Thanks Jeremy

Before you buy that though, I'd love someone to corroborate it. I'm not sure the exact path of brake fluid, and most failed masters that I've seen dump the fluid on the carpet of the interior, not the garage floor. I must've missed that the first time around. Can you actually see where it's leaking from?

FWIW, I think Rexmo had the info on the delrin pieces that fit for "rebuilding" the booster.

Jeremy

Took pics of the brake booster since I was attempting to rebuild it because of leaking ATF. The brake booster below is from an E23, but the operation is basically the same for all hydraulically boosted models (E23, E24, E28, early E32).

Brake booster leaking from the shaft connecting to the brake master cylinder:


Hydraulic brake booster disassembled:


Close-up of the brake booster plunger:


There are no part numbers for the individual parts. Some had success replacing the large black o-ring (38x3?) on the low pressure (near master cylinder) side. But mine was leaking on the shaft.

The seals at the ends of the plunger look like teflon/PTFE seals, probably with rubber o-rings underneath them. It works similarly to the M52/M54 VANOS, where the now infamous VANOS o-rings plasticize and lose their sealing ability.

I just wanted to post an update to this thread.

Working with my local hydraulics shop, I have identified 10 of the 11 seals for the Hydraulic Brake Servo Unit (BSU). This includes the rear shaft o-ring and teflon wiper, outer piston 0-rings and teflon seals, Cup o-ring, and the inner piston o-rings.

I am working on finding a supplier for the front shaft oil seal, but because it is integrated into the cup, its unlikely that there is a way to just attach it to the cup.
Plus, I have 3 cups and all three of the seals are in excellent condition so i get the impression that these dont get much wear. but maybe i'll sacrifice a cup in order to try to figure it out.

I'll post a follow up to this once i have driven it for a while and my rebuild is proven stable. (I dont want to get sued by anyone if their unit fails because they tried my solution).

I will say this, once I have a complete collection of seals for a BSU, I probably could rebuild one of these units in 15 20 minutes with hand tools. Its surprisingly easy.

FYI.....
I have a 73 E3, and started on this path when the BSU began leaking tremendously from the rosette (grub screw).

ruprectgern wrote:I just wanted to post an update to this thread.

Working with my local hydraulics shop, I have identified 10 of the 11 seals for the Hydraulic Brake Servo Unit (BSU). This includes the rear shaft o-ring and teflon wiper, outer piston 0-rings and teflon seals, Cup o-ring, and the inner piston o-rings.

I am working on finding a supplier for the front shaft oil seal, but because it is integrated into the cup, its unlikely that there is a way to just attach it to the cup.
Plus, I have 3 cups and all three of the seals are in excellent condition so i get the impression that these dont get much wear. but maybe i'll sacrifice a cup in order to try to figure it out.
I'll post a follow up to this once i have driven it for a while and my rebuild is proven stable. (I dont want to get sued by anyone if their unit fails because they tried my solution).

I will say this, once I have a complete collection of seals for a BSU, I probably could rebuild one of these units in 15 20 minutes with hand tools. Its surprisingly easy.

FYI.....
I have a 73 E3, and started on this path when the BSU began leaking tremendously from the rosette (grub screw).

[
I love it when somebody figures out how to repair a non-serviceable unit. If you need a a booster to dissect, PM me and I'll ship one gratis to further your research.

Excellent! Keep us posted.

Just keeping you updated.
I have figured out how to replace and identified every o-ring and seal in the Hydroboost unit. including the outer AND inner pistons.

The only seal that we cannot replace is the shaft oil wiper that is built in to the black plastic cup on the front of the BSU.
* - this is a custom molded part and i didnt want to screw with it.
* - I had the guy at the hydraulics shop look a couple over and he said that they must not get a lot of friction or use cause they were all in excellent condition
* - its just an oil wiper, its not a fluid seal so its not a critical part
With that said, if anyone wants to tear one apart and figure out if its possible, let me know what you figure out.

The "kit" costs raw, about $28.00.
I bought 3 "kits" (what i call my collection of parts) I have used all three. (I paid retail). Once I get a document out there you can look up these parts all on your own and buy them from whomever you wish. If you want me to rebuild yours, keep in mind that i'm not doing this for a living. send me a PM and ill get back to you.

So here is where I need your help
Unfortunately, my camera's SD card became corrupted immediately after recording the entire process, so I lost every picture. To compound the issue, I dont have any more units.

Here is my proposal to anyone who wants to contribute.
1 - Ship me a complete unopened BSU - must have the MC pushrod.
2 - Paypal me for the cost of the kit parts
3 - Ill rebuild your BSU and I'll record the process for a instruction guide.
4 - I'll ship the BSU back to you.

If this sounds good to you, PM me and let me know.

quote="Shawn D."]

bm0p700f wrote:Also what is the difference between the the brake booster in the M20 powered machines and the ones found in the M30's. I know they are of different shapes but the 528e/525e does not have the brakes bomb, why?

The M20 uses vacuum, the M30 uses hydraulics. The booster on the M20 stores vacuum (that's why there's a check valve), but the M30 booster does not and must rely on a separate hydraulic accumulator (the "bomb").

bm0p700f wrote:Specifically how does engine vacuum multiply braking force which I assume means increasing the pressure on the hydrolyic fluid.

Any pressure differential on an object will result in a force -- that's how all boosters work. In the case of vacuum-boosted brakes, the force of atmospheric air pushes a diaphragm into the vacuum-containing area. Also see:

http://auto.howstuffworks.com/auto-part ... -brake.htm[/quote]

Excellent response Shawn. I like it when we use words in the same way.


Did anyone send their booster for rebuild? How long will it take to turn it around?

Just to update for posterity since this thread may be a resource for others: As it turns out, my car was leaking ATF, not brake fluid as indicated previously. So it was the booster.

Patrick replaced the O- ring with one made of "nitril" (I think that was the name) and it has not leaked since. The size of the O-ring is 38x3 as Shawn mentioned.

I found this thread today and have traded a few PM's with ruprectgern. I'm headed down the road of the BSU rebuild in the next few weeks.
Where did you source the "nitril" O-ring?

Also, I have a used BSU that has been sitting in a plastic bag (not hermetically sealed) for the past two years. Do you think that the seals would still be good?

BTW, I wish more people would close out their threads with the final solution!

Thanks,
Andy

Not sure where Patrick McHugh sourced the O-ring. PM him and I'm sure he would be happy to share the info. He may even have some in stock.