H31 "Hydro Boost" power brakes + steering system

[shogun]

How I learned to stop worrying and love the bomb.
(From Stanley Kubrick's Dr. Strangelove)

This month I want to discuss a somewhat vintage BMW power brake system, the H31 "Hydro Boost" power brakes and steering system. BMW has always been a leader in technology and never was this more evident then when they introduced the H31 system in the E23 seven series cars in 1977. As is still typical the technology that was debut on the flagship Seven series became a proven design and used on various Five, Six and Seven series cars ending on the Eight series before being phased out with the last of the Eight Fifties in the mid nineties. Over the years I have received a great deal of calls and e-mails regarding the diagnosis and repair of this system. I will attempt to clarify some of the misconceptions and advise on accurate, straightforward diagnosis.

Unlike almost every other power assist braking system that uses engine vacuum to create the boost, the H31 system used hydraulic pressure from a special power steering pump to create the assist needed. This system allowed for more available assist that was highly controllable in a smaller package then any vacuum boost system available at that time. Soon after its introduction, BMW coupled its race proven BOSCH ABS technology to offer a braking system that became the performance standard that other manufactures where judged by.

Over the years the H31 system has caused a number of headaches for techs who where attempting to diagnosis this very sophisticated hydraulic system. Most of these headaches are due to a lack of understanding of the basic principles of operation. The original repair information involved the use of very high pressure gauges to test the pump output and the pressure controlled flow regulator known as the DS Regulator with it's attached accumulator better known as "the Bomb" because of it's cannonball shape. I have found the use of these gauges to be unnecessary in all but a few rare cases.

The most common failure of this system is the accumulator (the bomb). It is a round steel chamber with a high-pressure nitrogen filled balloon built inside it. On a normal working system, as the pumps hydraulic pressure builds up it moves fluid into one side of this sphere displacing the balloon and storing pressure and a volume of fluid. Over time the balloon loses its nitrogen charge causing the sphere to lose its spring affect of storing the pressure energy. The simple test of the bomb is to run the car for a minute to allow the pressures to normalize then shut off the engine and then pump the brake pedal until all power assist is lost and the pedal becomes hard. Typically a good accumulator will give you about eight pumps of the brake pedal before you loose assist. A bad one will be hard after one pump and the accumulator needs to be replaced. The second part of the test is to check the DS regulator. Again run the motor briefly to build pressure then shut it off. Now wait five minutes before applying the brakes. You should have at least half the pumps with assist that you had when you did the test the first time. If not the valves in the DS regulator are leaking down to quickly. The DS regulator cannot be serviced and would need to be replaced.

The common symptoms of a failed accumulator can be an intermittent brake warning light that can come on when the brakes are applied, or a too soft brake pedal that does not improve after bleeding the brake hydraulics. A good check for the brake hydraulics is to pump the pedal till all boost is gone, then if the pedal is still spongy then the problem is in the brakes. Other issues common to the booster system are leaking pressure switches at the DS regulator and leaks at the hydraulic booster. The brake booster leaking will vent power steering fluid (either ATF or Pentosin depending on the system) at a drain hole between the booster and the master cylinder.

The power steering pump supplying the pressure for this system has a maximum operating pressure of about 130 bar (1900 psi), however the working pressure for the H31 system is regulated at 35 * 57 bar and only needs about ten percent of the fluid volume that is needed for the power steering. With that in mind it is obvious that any problem with the pump pressure or volume would show itself first as a problem in the power steering.

To properly check the fluid level you must have the engine off, pump the brakes until the assist is gone then remove the cover from the reservoir. The fluid should be at the top of the screen. If not top off with the correct fluid for your car, most early cars used ATF but later BMW's went to Pentosin fluid, do not mix them and NEVER use brake fluid in the pump hydraulics. Brake fluid will destroy the seals in the H31 system and ATF or Pentosin will destroy the seals in the brake hydraulics. They are separate systems and do not share fluids, IF IN DOUBT ADD NOTHING. If either system is contaminated by the other it can be very expensive to fix and potentially dangerous.

My last set of problems, are usually listed as power steering complaints. This system uses a very high-pressure, high volume pump; any leaks in the system can be a big mess fast. Many of these cars develop noises in the pump and the pump is replaced only to find that the noise is still there! The problem is from air being pulled into the pump through loose hose connections on the inlet hose between the pump and the reservoir. The leak can be so small it will not lose any fluid but still allow air to be sucked in when the car is running. This problem is exaggerated by the fact that the filter in the reservoir has probably never been serviced and is plugging up with debris. Finally you must consider the condition of the drive belt, a belt failure will immediately cause a loss of power steering and the brakes will only have the stored pressure of the accumulator to assist you for stopping.

You can only imagine heading to the braking zone at turn three of BIR in your E28 M5 only to find that the belt broke. You would have no power steering and no power brakes if the accumulator were bad. No more M5, Bad day, really, really, bad day.

Until next time, Paul Dzimian, BMW CCA TSA, Shop Forman ,Motorwerks BMW

[shogun]

Bad Brake Master Cylinder, pedal soft,going to ground - solved

Here symptoms and remedy: (copied from a long thread)

Can pump the pedal and after several I get a nice firm pedal, but if holding pressure on the pedal, you can feel the pedal soft and slowly going almost to the floor, loosing stopping/holding power at idle, like at a stop light or stop sign. My first thought was a bad master cylinder, because of the way it is acting. This is because it is not really requiring more foot pressure, the "power" aspect is there, but, it is just not maintaining the pressure that was there in the first place, as the pedal keeps going closer to the floor. If it is the booster, I will fix it , but, it sounds more like it being the master cylinder to me. The brake fluid is down about 6mm from the full mark, and the Pentosin comes up to about 10mm from top after pumping till pedal gets hard.
--------------
After the testing, I replaced the master cylinder and now, all is well in the kingdom. Thanks to everyone that helped. Tony.
1990 750iL, 3/90 build date. 103400 miles. redcab
-----------------
Troubleshooting the brake system

Fault
Brakes pull to one side

Cause
a) Tyre pressure incorrect
b) Unevenly worn tire treads
c) Oil on pads/liners
d) Wrong type of pads/liners
e) Dirty floating caliper recesses
f) Guide bolts dirty or damaged
g) Rear wheel alignment adjusted incorrectly
h) Corrosion in floating calipers or wheel cylinders
i) No shock absorber action
k) Pad of one caliper worn
l) Pad glazed

Remedy
a) Correct tire pressure
b) Change or replace tyres
c) Replace brake linings, check for causes
d) Replace brake linings,check for causes
e) Remove and install/clean floating calipers
f) Replace guide bolts
g) Check wheel alignment
h) Remove and install, repair or replace floating calipers or wheel cylinders
i) Check shock absorber, replacing if necessary
k) Replace brake linings , Check floating caliper
l) Replace brake linings , Check floating caliper


Fault
Brakes excessively hot while driving

Cause
a) Vent bore in master cylinder clogged
b) No play between push rod and master cylinder piston
c) Swollen rubber parts due to use of wrong brake fluid
d) Vent bore in expansion tank clogged
e) Corroded floating calipers
f) Cross spring broken
g) Handbrake lever not released fully

Remedy
a) Overhaul master brake cylinder,replacing if necessary
b) Adjust push rod (in older cars)
c) Overhaul master brake cylinder,replacing if necessary
d) Clean expansion tank
e) Remove and install, repair or replace floating calipers
f) Replace cross spring
g) Check handbrake and handbrake cables ,repair if necessary

Fault
Poor braking effect in spite of great force on pedal
Brake-pedal travel normal

Cause
Brake pads oil-splattered or burnt; wrong type of brake pads

Remedy
Replace brake linings

Fault
Poor braking effect in spite of great force on pedal
Brake-pedal travel short

Cause
Brake booster malfunctions - engine vacuum

Remedy
Check brake booster system
- Check engine (valves, cylinder head gasket etc.)

Fault
Poor braking effect in spite of great force on pedal
Brake-pedal travel long

Cause
One brake circuit failed due to leaks or damage

Remedy
Leakage test of brake system

Fault
Brake pedal can be pressed down softly and springily

Cause
a) Air in braking system
b) Insufficient brake fluid in expansion tank - refer to a)
c) Overheated brake fluid - vapour lock due to excessive water content in brake fluid or excessive brake loads

Remedy
a) Top up or change brake fluid
Bleed brakes
b) Top up or change brake fluid
Bleed brakes
c) Top up or change brake fluid
Bleed brakes

Fault
Brake is bled and readjusted, but brake pedal can be pressed down too far

Cause
a) Primary cup in master cylinder damaged
b) Separating cups on floating piston of tandem-brake master cylinder leak
c) Leak in braking system

Remedy
a) Overhaul or replace brake master cylinder
b) Overhaul or replace brake master cylinder
c) Leakage test of brake system

Fault
Uneven pad wear

Cause
a) Wrong type of pads/liners
b) Dirty fist caliper recesses, damaged caps
c) Corrosion in floating calipers or wheel cylinders
d) Swollen rubber ring for piston control

Remedy
a) Replace brake linings
b) Remove and install, repair or replace floating calipers or wheel cylinders
c) Remove and install, repair or replace floating calipers or wheel cylinders
d) Remove and install, repair or replace floating calipers or wheel cylinders

Fault
Brake pads worn at angle

Cause
a) Wheel-bearing play excessive
b) Brake disc not aligned with fist caliper
c) Corrosion in floating calipers or wheel cylinders
d) Angular brake-disc wear
e) Pads worn to less than minimum thickness
f) Spring force insufficient
g) Guide bolts damaged

Remedy
a) Replace wheel bearings
b) Check floating caliper installation
c) Remove and install, repair or replace floating calipers or wheel cylinders
d) Grind or replace brake discs
e) Replace brake linings
f) Replace spring
g) Replace guide bolts

Fault
Seized brake pads, pad does not move off brake disc

Cause
a) Dirty fist caliper recesses, damaged caps
b) Corrosion in floating calipers
c) Vent bore in master cylinder clogged

Remedy
a) Remove and install, repair or replace floating calipers
b) Remove and install, repair or replace floating calipers
c) Overhaul or replace brake master cylinder

Fault
Brakes squeal or rattle

Cause
a) Wrong type of pads/liners
b) Dirty floating caliper recesses
c) Spring force insufficient
d) Brake disc not aligned with fist caliper
e) Brake-disc runout
f) Excessive thickness deviation within braking surface
g) Brake drums eccentric
h) Knocking brake drums
i) Dirt and dust in brake drums
k) Liner wear excessive or one-sided
l) Rust edges on brake discs
m) Pad loose
n) Wheel-bearing play excessive

Remedy
a) Replace brake linings
b) Remove and install/clean floating calipers
c) Replace spring
d) Check floating caliper installation
e) Check brake discs for runout, replacing if necessary
f) Measure brake disc thickness
Grind or replace discs
g) Grind or replace brake drums
h) Grind or replace brake drums
i) Clean and check brake drums
k) Replace brake shoes
l) Grind or replace brake discs
m) Replace brake linings
n) Replace wheel bearings

Fault
Brake-pedal dead travel excessive

Cause
a) Wheel-bearing play excessive
b) Brake disc not aligned with fist caliper
c) Brake-disc runout
d) Excessive thickness deviation within braking surface
e) Brake system leaky
f) Air in braking system
g) Wrong type of pads/liners
h) Automatic slack control of brake shoes does not function

Remedy
a) Replace wheel bearings
b) Check floating caliper installation
c) Check brake discs for runout, replacing if necessary
d) Measure brake disc thickness - Grind or replace discs
e) Leakage test of brake system
f) Bleed brakes
g) Replace brake linings or Replace brake shoes
h) Remove and install brake shoes , repair slack control

Fault
Jammed piston in brake caliper

Cause
a) Dirty fist caliper recesses, damaged caps
b) Brake disc not aligned with fist caliper
c) Corrosion on pistons in floating calipers or wheel cylinders

Remedy
a) Remove and install, repair or replace floating caliper
b) Check floating caliper installation
c) Remove and install, repair or replace floating caliper or wheel cylinder

Fault
Pulsating effect on brake pedal

Cause
) Wheel-bearing play excessive
b) Brake disc not aligned with fist caliper
c) Brake-disc runout
d) Excessive thickness deviation within braking surface

Remedy
a) Replace wheel bearings
b) Check floating caliper installation
c) Replace brake discs
d) Measure brake disc thickness
Grind or replace discs

Fault
Handbrake effect insufficient

Cause
a) Brake shoes oil-splattered
b) Excessive dead travel between brake shoes and brake drums
c) Excessive dead travel in cables
d) Cables adjusted incorrectly
e) Corroded transmitting elements

Remedy
a) Replace brake linings, determine cause
b) Adjusting handbrake
c) Adjusting handbrake
d) Adjusting handbrake
e) Remove and install handbrake and expander locks
Check Bowden cables, replacing if necessary

Picture of brake master cylinder E32 750 and also older 735 with the H31 hydraulic brake booster system, they use the same brake master unit
https://www.bimmerforums.com/forum/s...-brake-booster

[htrdbmr]

I guess I went backwards when I built my car, the vacuum booster for the 735 wouldn't clear my valve cover, so I switched to a hydro boost from a Mustang. Never had a moment's trouble.

[shogun]

Yes, that is what most people do when they for example plant in E30 the M70 engine, H31 hydraulic brake booster requires much less space.
Besides the occasional small leak between brake booster and master there is in my opinion also no big trouble. Besides that, there are now some people which offer rebuilding of the H31 brake booster unit for the older E24, E32, very old E32 735 (1987/88) and of course all E32 750 and E31 850. For example a member from the E23 forum J. Stratton has a service for rebuild or sets https://picasaweb.google.com/1135658...nZ_Z7f6JrUyQE#
He writes: For the following cars, with Hydraulic brake servo booster, and ABS. e24, 628,633,635,m635,m6,L6; e28, 525,528,533,536,m535,m5; e23, 728,730,732,733,735,745; e32, 730,735,750;
I now have of the interior components for the rebuild of the brake servo booster. I installed these new parts into my M6 over the weekend. The road test was a success, all of the parts work as they should! The new up-graded kit includes the 6 internal rubber o-rings, a 9.5mm I.D. shaft seal. 7 pieces total. I have also up-graded the Picasa Link, I took more photos and added more captions to show the correct method that I used in the assembly of the booster. Link to the DIY, and the current pricing, and the step by step instruction, of the booster repair.
BOOSTER REBUILD SERVICE
I am now offering to rebuild your booster for you, using my 6 piece repair kit. For the e23,e24,e28,e32-735. The price is $80.00 + shipping back to you. Cont USA only, USD. If you send me your address, I can give an accurate shipping quote.
Via FedEx Ground home delivery, with tracking. I will then send you my address. Just send your booster to me, with a $ check. You insure for $190.00, PayPal add 4% . On the e32 750, please refer to my DIY link.

REBUILD SERVICE WITH CORE
I also have a couple of spare rebuilt boosters, and I can send one to you. And you return your old core to me, return shipping on you. Price is $80.00 + ship + $75.00 core. Cont USA only, USD. You insure for $190.00. When the core is returned, the Core deposit will be refunded via PayPal. PayPal add 4%. On your e32 750, please refer to my DIY link.
http://www.bimmernut.com/forum/showt...e32-(735-only)

Other articles: http://blog.fcpeuro.com/2014/05/22/f...oboost-system/
By Bryan McPhail : How BMW’s Hydroboost Braking System Works
In the 90′s, power brake assist was not vacuum assisted, rather it was provided by a hydraulic system known as the ATE H31 Hydroboost in BMW’s high end cars up to the 850Ci. I’ve read in places that BMW chose this for the V12 powered 850i and 750iL because of the difficulty in fitting the larger vacuum booster into the very cramped engine bay. Certainly those engine bays are tight and hard to work on but I suspect BMW were just as interested in using the performance of this unit in their flagship cars.
There are 3 main components to this system – the pressure accumulator, the booster and the regulator.
The accumulator is often known as the “Bomb,” not just because of it’s shape but because of the high pressure it can contain. The bomb is filled with a nitrogen compartment that gets compressed by the hydraulic fluid. This ‘charge’ remains in the bomb even when the system is turned off. The bomb can release a large amount of pressure through the fluid very quickly when required. Fluid from the pump flows to the regulator where it is diverted to both the bomb, and the power steering box (the same pump drives both systems). You can see two electrical connectors on the regulator – these attach to pressure sensors – the top one monitors the pressure coming from the pump, and the side mounted one monitors the much higher pressure at the accumulator bomb. The driver can then be alerted to failure in either of these systems.
The system still works even if all engine power is lost

Fluid from the accumulator travels to the input of the booster unit (top right port in picture), there is also a return line to the reservoir (top left port). Simply put, when the brake pedal is pushed in (rod on the right hand side) the hydraulic pressure assists in pushing the output rod (would emerge on left hand side of picture) which acts on the brake master cylinder input rod. One of the performance benefits of the system is that the hydraulic force is constant – it doesn’t vary according to engine conditions like a vacuum system could. A safety benefit is that the system still works even if all engine power is lost – the rule of thumb is that the system should provide 15-20 power brake assists even when the engine is off. Any less and you should suspect failure in the accumulator.On cars 15-20 years old (or younger) the main problem with the system is leaks – the seals inside the booster can give up, leaks can occur at both pressure switches, and the accumulator itself can fail. (a leaking diaphragm allowing nitrogen to leak out and mix with the hydraulic fluid) Luckily all of these parts are still available to buy new.

[shogun]

And here info on the brake master and how to repair it, we disassembled a unit http://www.bimmerforums.com/forum/sh...t=brake+master

[todor]

Great info. Can we add one more possible fault to the list? Pressing the brake pedal hard (even at idle, when stopped) makes a hissing noise, as if something is leaking from a hydraulic system. Hissing stops when I stop pressing the pedal that hard. What could it be?

[shogun]

single parts of a disassembled H31 brake booster https://www.shiftbmw.com/model/e32/b...brake-booster/
Nice color section design drawing http://twrite.org/shogunnew/files/br...rsection01.jpg

[master6]

Shogun,


Thank you for your fine analysis, of the Brake Booster.

This applies to my rebuilt, H31

1. My Hydraulic Brake Servo Booster, Repair Service,

2. My SLS (self leveling suspension pump). Repair Service,

3. My PSP (power steering pump) Repair Service.

I have a my links located in the parts for sale section here in BF.

here is the link : http://www.bimmerforums.com/forum/sh...-Sept-Oct-2016


Best,

[shogun]

More info:
34331159752 is the brake proportioning valve on the pressure Regulator/DS regulator.
34331150922 is a pressure warning switch/Brake pressure differential switch
34331159752 controls a small flow of approx. 0.7 liter/minute for the hydraulic brake accumulator (DS) and loads/keeps it at a pressure of 35 - 57 bar.
So if the brake proportioning valve is not functioning as per spec., the brake bomb has a different load.
Download this, German technical desciption of the H31 system, including testing of the single components. http://www.bimmerboard.com/members/s..._H31_00000.pdf

and here I have a function diagram how it works, download it:
Hydraulische ATE Bremskraftverstärkungsanlage / hydraulic brake booster system H31
1. Vorratsbehälter / storage container
2. Tandemhauptzylinder mit Ausgleichsbehälter / tandem main cylinder with expansion tank
3. Hydraulischer Verstärker/ hydraulic booster
4. Druckgesteuerter Stromregler / pressure controlled flow regulator
5. Hydrospeicher / hydraulic storage
6. Lenkungspumpe / power steering pump
7. Servolenkung

Ausgangsstellung = normal position/when not breaking
Teilbremsstellung = partial braking
Vollbremsung = full braking

this is the important point: Druckgesteuerter Stromregler mit Hydrospeicher = pressure controlled flow regulator with hydro storage
Speicherladestellung = brake bomb loading position
Umlaufstellung = circulation position
Hydr.Bremskraftverst_D[1].pdf

[unoot]

Jay, can you provide me with details (shipping, payment, turnaround, etc) for an E31 brake booster rebuild. Thanks
Uri

[master6]

Uri,

Is this for your e32 740?
Can you give me your build month and year, also your zip code.
And I can quote the return shipping via FedEx.
jay_c_stratton @ hotmail.com

[rondwyer]

I have a peculiar issue with my brakes on a modified 77 E-12 with an H-31 system. It has been on the car for a couple years now and I have recently started to drive it again after passing smog. I was surprised by a loss of braking one day via the pedal effort. So, I replaced the master cylinder. All seemed well until a few weeks ago I got the same problem, pedal goes down and I am forced to use the hand brake and downshift. I parked the car, looked for leaks and all was dry with a full fluid reservoir. Leaving it sit for about 30 minutes I decided to take side streets and limp it home. Surprisingly, the braking effort was back to 100% and has not faulted as described since. I am not in a position to trust this car at present. The kit came from an 89 7 series so I am wondering if I have seal degradation because of running ATF in the power steering system, not sure if Pentosin was what the 7 would have used. I believe there is a solid mechanical connection through the H31 to the master cylinder so based on this episode, I think I may have a faulty Master Cylinder, is this likely the case? Just want to make sure I am pursuing the most likely culprits.

[shogun]

You replaced the master cylinder with a new or used one? If used, it might be the master cylinder
https://www.bimmerforums.com/forum/s...t=brake+master
https://www.bimmerforums.com/forum/s...ight=H31+brake
Here a pic I posted of a disassembed master cylinder, there are 4 seals inside, and they might be damaged https://www.bimmerforums.com/forum/s...t=brake+master
There are remanufacured brake master cylinders available, easiest, not that expensive.

[Timthefinn]

I'd like to convert my 2000 z3 (E36/7) from vacuum booster to hydrobooster. Would one of these boosters work for this, or is there a better way to do this for a Z3?

Thanks.

[unity]

Your main concern would be where to mount all the hardware, specifically the hydraulic pump on the engine.

[shogun]

E30 owners which install a bigger engine like to use the H31 system, as it requires less space than the vacuum unit.
They even use the pics and info from my website http://www.e30tech.com/forum/showthread.php?t=98434

[Timthefinn]

Your main concern would be where to mount all the hardware, specifically the hydraulic pump on the engine.

At the risk of drawing ridicule... I'm converting the car to electric, so have to buy an electric hydraulic pump for the power steering and figured that this would be a small enough additional load to use it for this too. Am I assuming too much here? Otherwise, I need to buy and install a vacuum pump which is noisy (especially for an EV) and a good quiet one is kinda expensive. I'd rather put that money and effort to a hydraulic booster.

[Timthefinn]

E30 owners which install a bigger engine like to use the H31 system, as it requires less space than the vacuum unit.
They even use the pics and info from my website http://www.e30tech.com/forum/showthread.php?t=98434

Thanks. BTW, I'm in Yokosuka Japan at the moment... But my project car is in Silverdale Washington. Do you think this thing would just swap out on the backside of my master cylinder, or will the two not work together and I need to get a booster/master cylinder set to replace it with. Then, is it possible to use the ABS of the car? If need to simplify it to no ABS, I can probably live with that. Just trying to determine the scope of the job...

[shogun]

Better ask the experts which have done it in such an E30 forum, I have never done it and I do not know the details + dimensions of a Z4 brakes. Even on E32 there are 2 different brake boosters and the ones on E23,E28, E24 are different, the push pin inside has a different length. I can give you a link where someone has converted from H31 to vacuum, so you can see what is needed the other way round http://bmwe32.masscom.net/maxf_websi...er/booster.htm
Be reminded that a new brake booster H31 sets you back approx. $1500, a pressure accumulator used up to 12/1990 34331162055 was about $400 and it no longer available, we usually modify Febi accumulators to fit into older E32 750 with one connection nowadays, newer ones have 2 connections. Old H31 brake boosters leak, there is only one expert here on the forum which repairs them, that is master6 . E32 735 with H31 use a different version than the E32 750. Even these are not interchangeable and I have my doubts that it will be plug and play on an Z3. I assume there are for sure cheaper ways to modify the Z3, maybe the idea with the vacuum pump for example.

If you want to convert the car to electric, study for example the Toyota hybrid brake system. My wife has a Auris Hybrid, there the brake shoes last 100000 km, as the system uses usually the generator to produce new power for the electric system instead of the brake rotors and shoes. google for How the Toyota Prius Regenerative Braking System works. What is Toyota Regenerative Braking?
Regenerative braking employs an electric generator to recover kinetic energy that is otherwise lost when you step on the brakes. A tremendous asset to the Toyota Prius design, regenerative braking uses a mechanism within the engine design to recapture kinetic energy every time you press the brakes. Rather than use conventional brakes, a regenerative braking system puts the engine into reverse to conserve energy and create electricity. This electricity is then stored in the batteries to run electrical systems.

[Timthefinn]

All great info. Thanks. I was curious on price, so that gives me info to chew on. Another solution I found out about is a Bosch iBooster which is an electric booster designed for electric and hybrid cars. Not normally sold alone to the man on the street, but maybe can find one being parted from a Tesla. Nice to have options... Thanks again!

[unity]

Electric conversion? Sweet! I would love to do such a project sometime.

[shogun]

And here a detailed technical description in German language Hydraulische Bremse H31, includes function description of the complete system, function description of the hydraulic brake booster, function desciption of the brake proportioning valve, testing of the system. For general function test
Bei abgestelltem Motor Hydrospeicher durch ca. 20 Betaetigungen des Bremspedals mit dem einer Vollbremsung entsprechenden Kraftaufwand den Hydrospeicher entleeren
= with engine off push the brake pedal with the same force like in emergency brake about 20 times to empty the brake bomb

Bremspedal geringfuegig belasten, Motor starten, dabei muss sich dass Bremspedal spuerbar tiefer senken. Sollte dieser Effekt nicht eintreten, kann die Lenkungspumpe, der DS-Regler, der Hydrospeicher oder der hydraulische Bremskraftverstaerker defekt sein =
= Push down the brake pedal slightly, start the engine, then the brake pedal must do down further. In case this does not happen, the flwg parts can be defective: power steering pump, the DS-regulator oder the hydraulic brake booster unit

Diagrams of the system also shown.

[TM 850i]

Hi all. New to the 8 world! I picked up a 92 850i with MK Motorsport aftermarket. So far I have replace the rad, put in wokke chips ( wow - big difference), and steering gearbox. Noticed this post and I am trying to find somewhere to purchase an H31 brake hydrabooster as a rebuild kit. Canada is pretty thin on parts.
Tackling a/c soon as well ( pretty sure the same issue will come up.
Any help would be much appreciated.
T

- - - Updated - - -

Sorry typo. Complete unit and a seal kit.

[master6]

TM 850i

Sending message

[nasiroddin]

hope the forum can help my frustration on my bmw e28 m5 the brakes binding sometimes all 4wheels sometimes just the front ones this happens when the car gets hot brake peddle becomes hard. After a while brakes ok did the brake bomb test by starting the car for a minute then shutting off the engine till all assist is gone pumped the peddle 8 times peddle felt normal, i removed the master cylinder from the brake booster and in the chamber i found power steering fluid dripping from u small pipe on the booster please help i also had all 4 calipers renewed with orignal bmw kits