E31 wooden brake pedal feel a possible solution??

[merlin840]

I have bought/swapped an 90 850i for a 95 840ci + cash it has been an interesting comparison but anyway the 850 has horrible wooden brakes unlike the 94 840ci 6spd i have which are wonderful and the 850csi and the 840ci i just swapped were somewhere in between i was just about to replace the brake bomb i have a new one to fit but i decided to replace the pressure switches
BMW pt/No 34331159752 and 34331150922 and the feel of the brakes has improved dramatically I also note in the TIS BMW recommend replacing these switches when you replace the accumulator/bomb unfortunately since i have replaced those switches everytime i use the brakes the red brake symbol flashes briefly when i use the brakes i assume that the bomb is indeed buggered but the change from replacing those pressure switches was quite surprising

they are fortunately cheep too

[dragon850]

I have bought/swapped an 90 850i for a 95 840ci + cash it has been an interesting comparison but anyway the 850 has horrible wooden brakes unlike the 94 840ci 6spd i have which are wonderful and the 850csi and the 840ci i just swapped were somewhere in between i was just about to replace the brake bomb i have a new one to fit but i decided to replace the pressure switches
BMW pt/No 34331159752 and 34331150922 and the feel of the brakes has improved dramatically I also note in the TIS BMW recommend replacing these switches when you replace the accumulator/bomb unfortunately since i have replaced those switches everytime i use the brakes the red brake symbol flashes briefly when i use the brakes i assume that the bomb is indeed buggered but the change from replacing those pressure switches was quite surprising

they are fortunately cheep too

Your brake accumulator is dead!

[merlin840]

yes i believe so but it will have to wait until at least tomorrow to replace it i was mostly interested in the difference changing the pressure switches made to the feel of the brakes and how the light that flashes on did not appear before the change of the switches
the aim was to replace the accumulator anyway but i had some spare time today to do something but not enough time to change everything whereas when the accumulator failed on the 840ci 6spd i only changed the accumulator and none of the pressure switches and i noticed very little difference except the light on the dash board went out

[merlin840]

I have now fitted the new accumulator that i had bought and can now report that the brakes feel 100% better not quite as good as my 4 pot 840ci but that could be pad material and/or disc rotor condition the big difference did appear to come from those pressure sensors that i replaced

[BMSman]

I'm curious how do pressure sensors affect the braking performance? I did not realize they controlled anything. I thought just ot indicate low hydraulic pressure. Are you referring to the pentosin hydraulic pressure sensor or something else?

[TedBobo]

I'm curious how do pressure sensors affect the braking performance?

Me too!

[merlin840]

I would assume that the one of the pressure sensors will turn a solenoid or similar on and off to charge the hydraulic accumulator(brake bomb) perhaps it is just operating at a low pressure = less power assistance and the other is a differential switch which will be measuring the pressure difference and at a guess tells you that your brake bomb is buggered mine had no warnings until i changed these sensors the pedal/brakes felt horrible and occasionally had no power assistance(scary)
TIS recommends you change these at the same time as your accumulator and since at least 1 of mine were not working and the improvement it has produced its worth a go

[shogun]

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...mskraftverst_D[1].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
http://www.bimmerboard.com/members/s..._H31_00000.pdf

H31 "Hydro Boost" power brakes + steering system
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

once was linked on http://www.northstarbmw.org/

[merlin840]

well my assumptions were close
excellent just the explanation i had hoped for thank you mr shogun

[Gary Knox]

Shogun,

THANK YOU - I now understand exactly how this system works, and how to maintain it. Should be mandatory reading for ANYONE who buys an e31!!

Gary Knox

[kiwiCSi]

Agree - shogun's post is a great reference text, and should become a "sticky" or at least be placed in the 'Parked: E31 Information' thread.

[toomanyparts]

This was helpful. Thanks again Shogun and merlin840.

[E36Ron]

Hey Shogun:

Thanks for the wonderful post on the H31 system.

Now here's the challenge, on my wonderful 1997 850Ci: the brake pedal is too low and too soft; and the amount of braking pressure applied to the wheels is insufficient -- stopped at a traffic light with normal resting foot pressure on the pedal (at its lower than usual position), the pressure ebbs and the car starts to creep, requiring additional foot pressure.

When I acquired the car in 2016, the bomb was replaced before delivery. Every test I have done since demonstrates tremendous braking power in a panic stop; there do not seem to be any of the usual signs of a leaking master cylinder (except those are hard to separate from problems within the H31 itself on this car). The brakes are fine except for the lower position and slightly greater pressure required. BUT SOMETHING IS NOT RIGHT.

What controls the position of the pedal at rest? What controls the amount of pressure applied to the wheel pistons at a standstill? What could be leaking, or otherwise cause a gradual ebbing of pressure at a standstill?

I need to solve this problem before I take this car out to the Bonneville Salt Flats in Utah!

Best regards,

-- E36Ron

[shogun]

brake booster is leaking or the brake master has defective seals? Or the brake proportioning valve on the pressure Regulator/DS regulator?
When did you change the last time the brake fluid and make brake bleeding? If too old, moisture......
Here I posted more info on the H31 system and fault findings
https://www.bimmerforums.com/forum/s...teering-system

[E36Ron]

Thank you, Shogun, I will go check all that out!

-- E31Ron

[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. I assume you have to retype all the German text and then google translate or better is https://www.deepl.com/translator

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 go down further. In case this does not happen, the flwg parts can be defective: power steering pump, the DS-regulator or the hydraulic brake booster unit

Diagrams of the system also shown.

[Money Pit]

Hi All,

Also consider replacing the flexible brake lines. I used a 6 piece "Exact Fit Stainless Steel" kit from ECS (ES#2602382) on my 840 a few years ago and it made a huge difference in both pedal feel and braking.

[E36Ron]

Dear All:

Thank you for your suggestions. The very first post in this thread is Paul Dzimian's classic "Dr. Strangelove" article about how to diagnose the H31 braking system in BMWs.

Here are the results of his tests on my car in question, which I should emphasize has great working brakes -- it's just that the pedal is too low, and there is too much play in it for comfort:

. General Function: Start engine, stop; pump brake till hard, hold pedal, re-start engine. Pedal should go down. It does. Check.

. Accumulator Test: Start engine, stop. Pump brake till hard, count number of pumps till hard. Count: 18 presses. Check.

. DS Flow Regulator: Start engine, stop. Let sit 5 minutes, count number of presses till hard. Count: 11 presses. Check.

. Hydraulics: Start engine, run, stop. Pump brakes till hard. If does not get hard, hydraulics bad. Pedal gets metal-to-metal hard. Check.

. Power Steering Pump Pressure: if bad, power steering would suffer first. Power steering great, lock-to-lock with one finger. Check.

The Accumulator is new within the past 3 years and 5,000 miles (and the Pentosin and filter changed at the time of that installation), and all the brake lines have been replaced with steel braided lines in a set from ECS Tuning (highly recommended).

It's not the calipers, it's not the pads, it's not air in lines (bled with MoDiC to control ABS), and if the "Hydraulics" test above is to be so interpreted, it's not master cylinder.

To my mind, this leaves only the mechanical pedal linkage to the brake booster plunger, which Master6 says was pre-set at the factory, and should never be changed.

Perhaps it's been butchered by some idiot, or something broke there. Any information about the linkage, and particularly the factory adjustment, before I get under there would be greatly appreciated.

-- E36Ron

[shogun]

already checked the 4 bushings on the brake pedal? # 9 http://bmwfans.info/parts-catalog/E3..._light_switch/
I once had some of them broken and that was also a strange pedal feel

(edit: 2 for brake pedal, 2 for clutch in manual cars)

[E36Ron]

Hi Shogun:

Thanks for the diagram with bushings #9, but I only see two for the brake pedal, not 4. The other two are for the clutch pedal, not present in automatic cars.

Following your cue, I found the diagram for the booster servo, and put together a combined conceptual picture of how they connect:

Attachment 681292


It looks like the "brake pedal retainer spring," though small in the picture, must be the spring that returns the pedal to the normal "unapplied" resting position.

The "fork head" is threaded onto the booster plunger, through an unidentified bracket marked #3, whose purpose is not clear, and which is not sold separately.

With the brake pedal and the booster plunger rigidly connected (adjustable through a screw thread in the fork head) there must be some play in the booster plunger before pressure is sent to the brakes, otherwise the pedal could not come back up to a resting position and still have some give in the travel before brake application took place.

So it's possible that in this case the plunger is unscrewed to some extent, giving extra play and dropping the pedal down. Then screwing it back up further into the fork head might bring the pedal up a couple of inches (without too many turns of the screw thread since the pedal is far out on the radius compared to the fork head attachment pin).

If this does not happen, and there is no physical damage to the supporting bracket or bushings, then I would be inclined to suspect the internals of the booster unit.

Could someone here with more experience than I comment on my logic?

(And does anyone have any ideas about the mystery bracket between the fork head and the booster plunger? -- see diagram.)

I can't see any other way to set the pedal height correctly at the factory in the fist place without this adjustment. This has to be what controls the pedal height, all other components being assumed OK, which seems to be the case since the system otherwise works perfectly. (In which case, how it got unadjusted, who knows?)

And if there is a danger in making this adjustment, assuming everything else is in working condition, what might that be?

Thank you all for any help!

-- E36Ron

[shogun]

first of all check the brake pedal bushings, I had them broken and the pedal really reacted strange, one side of the bushes cpl. disappeared and the other side was just small plastic pieces.
Here is some more info on the method of operation of the hydraulic brake servo unit and a nice diagram he took from my website "Funktionsschema der hydraulischen Bremskraftverstaerkeranlage" http://bmw8.us/Brake%20Booster%20Rebuild.htm

[Max Lumens]

Hi Shogun- Been a while; thanks again for the use of the old ATE catalog diagrams, and the new German text you just posted. I tried to OCR the .pdf and translate, but no joy.

I've also seen the bushings disintegrate on both pedals, with sloppy feel. @Ron, I've never seen that unidentified bracket installed on a car. It shows up in the ETK but grayed out, no PN, and it's not shown on any other models that used the H31 system. Possibly something that was dropped from production early on?

It's not verboten to adjust the linkage to the input shaft of the booster; just be sure to leave a little slack so that the piston can return completely, or your brakes may drag. IIRC there's a spec for it in the manual, but a couple millimeters is plenty. And make sure the pedal is coming all the way back up to the stop courtesy of the return spring. The input shaft is a wobble mount inside the piston, not rigidly perpendicular, so it can accommodate the arc of the pedal.

[clockwork]

and the amount of braking pressure applied to the wheels is insufficient -- stopped at a traffic light with normal resting foot pressure on the pedal, the pressure ebbs and the car starts to creep, requiring additional foot pressure.

-- E36Ron

this exactly happens to my car and accidently, one time, while I was looking at my smart watch, I crept forward and hit the vehicle ahead of me.

I've ALWAYS had the issue of when I was to perform the task where with car turned off, it takes me 45 brake pedal pushes till my pedal goes solid. NOTHING remotely close to the 20 pedals everyone else comments about.
While driving, I fel there is a sheet of parchment or wax paper between my rotors and pads and I have to jam my foot down to achieve any real stopping power and I've tried the "sudden stop" test to check if I have that moment of "oh crap" before the brakes kick in, and I do not get that. I too have swapped out to stainless Steel brake lines years ago and while that helps, the brakes have always been a horrible performer on my car.

[clockwork]

unrelated, yet somehow related neat pic from my archives...PS Hydraulic System.jpg