1996 E31 850CSI Loss of brakes

[stuey]

Hi folks im new to the beemer forum so hope you get to read this lol.

I have owned my 850csi for about 4/5 years now and its my sunshine toy, she is a 96 model on an "N" reg with the 6 speed transmission , and what a beast !!.

That was until it all went petetong !!
some time ago the rear hydraulic steering went into safe mode and locked up thus displaying "safe mode" on the onboard computer , i never had found out why but the car still drove.
Then 6 months later the "ABS" light stays on on the dash despite changing a faulty sensor on the DSF.
Then a few months later i was driving the car on a lovely sunny day to my folks for dinner when all of a sudden i lost the braking on the car and the brake pedal went flat to the floorboards with hardly any resistance and no brakes !!.not sure if any of these faults could be linked to the same underlying problem as all 3 faults still remain.


When the engine is switched off and the brake pedal pumped it becomes solid as most cars as it should, but when the car is started and after 5 or so pumps of the pedal its back to the floorboards with no resistance and the car barely stops at all, i can hear what sounds like a air wooshing sound when opperating the pedal but ruled out a split vacuum diaphram as the "8s" dont have them its a hydraulic piston i beleive.

I cannot find any leakage of brake fluid nor any leaks from the secondary hydraulic assistance system on the car. I belive this hydraulic system aids the power steering / rear steering and brake servo cylinder via the brake bomb ???and fills up in a resovior tank under the bonnet.

I just dont know where to start , i have my suspicions as to the cause but am only guessing and reluctant to strip the master cylinder assembley to find its perfectly fine.

can anyone help me as its been sat in my garage for over a year now lookin very sorry for itself !

Thanks in advance Stu.

[avuse31]

Hi Stu. Not sure about your current total loss of brakes but I had the same problem with the rear steering on my last 850ci. It turned out the rear wheel steering was on the same fuse as the A/C condensor cooling fan. The fan had seized during the winter lay up and when it tried to kick in when I started to use the car it blew the fuse taking out the 4 wheel steering as well so check your fan isn't siezed and the fuse is o.k. With the faultly ABS is the traction control out of action as well, if it is it is probably the dreaded faultly ABS pump. If so you will need to get the pump rebuilt or get a secondhand one. The brake failure sounds like the master cylinder seals have gone but that is probably too simple. I know this doesn't really answer you question but they might be worth checking.
Cheers Simon

[stuey]

Thanks Simon for your prompt reply, il check the fuse tomorrow and the fan. As for the ABS yes i have lost traction control aswell ,the traction control light will turn on and off whilst the car is stationary but stays on when moving, the abs light however shows up after driving a few hundred yards and above 10mph ?!.
Now you have said the seals in the master cylinder that confirms my suspicions a little more as thats my theory too , im assuming the hydraulic brake servo is what causes the brakes to be stiff with the engine off as the hydraulic fluid would not be able to move from the cylinder but then it shows up the fault as soon as the brake servo kicks in.
Is it possible the brake fluid has lost its viscosity do you think ?
One other thing iv noticed is the power steering feels notchy as if there is air in the hydraulics of the steering rack ?
these cars are so complicated , im quite a competant mechanic and have re-built the engine in my vectra sri but this car scares the sh#t out of me as its so advanced and computer orientated.

Regards Stuart.

[avuse31]

Hi Stuart, I am sure your ABS problem is the pump, I have had two 8 series with this problem. Usually arises in cars that haven't been used much, I believe the motor in the pump sticks. A new pump is very expensive (circa £1600 I think) you can have them reconditioned but make sure you go for a reputable firm if they just give it a quick clean out and a coat of paint you will be back to square one in a few months.
The brakes/PAS/self leveling all use the same fluid. In the engine bay on the RHS as you look in from the front is a container about 4" across, if you spin the plastic nut off and then remove the top you should see a gauze. This container provides fluid for the PAS and also the hydraulics for the brakes (instead of a servo). To check the level, with the engine turned off, pump the brake pedal about 10 times and the fluid should be at the bottom of the gauze. IMPORTANTLY the fluid is CHF11S, it is a light oil pale green in colour. If you take a sample of what is in your resovoir and it is black or red or anything other than pale green the fluid is either very old or someone in the past has put ordinary PAS fluid in there which is much heavier and will not work properly. There is also a filter in there which need replacing (only avail from BMW I think). There is a pump on the engine (like a PAS pump) which powers this system. There is also a valve above the brake fluid resovoir that distributes the fluid, it has two electrical pressure switches screwed into it.
I don't think the state of your brake fluid has any bearing on your present problems. If the fluid has not been changed in the last few years it will contain a lot of water which might give you brake fade at high speed but nothing else.
I would go for the master cylinder first as faulty hydraulics (i.e. servo system) would give you a constant hard pedal. The ABS/Traction is most certainly the pump. The notchy steering maybe the fluid or faulty hydraulic pump.
Cheers Simon

[Talyinka]

By the sound of it we have several unrelated faults here.

The important one is of course the brake system fault and it sounds a bit weird from the description. Even if you lose all power assistance in the braking system it will work. It has two separate hydraulic circuits and the only way I can think of generating what you describe is if one of the circuits have been out of order for some time without you noticing and the second system subsequently malfunctioned.

Now, if as suggested, the master cylinder seals/pistons have gone the effect often is to push brake fluid into the central hydraulic system, leading to fluid loss and an associated warning, from the brake fluid reservoir on top of the brake cylinder. So the first step is to see whether fluid is missing from either of the brake fluid reservoir chambers (there are two connected chambers, one for each brake circuit). If you have brake fluid in the central hydraulic system you need to flush it out and replace with the appropriate Pentosin central hydraulic system oil type, which for a '96 will be the synthetic CHF 11S. (I have not seen any alternative brands guaranteeing compatibility with the Pentosin fluids although some claim to fulfil BMW's requirements).

The normal repair manual contains a whole raft of test procedures for the various hydraulic systems, as well as drawings explaining how they work. In general, the 8-series cars require scheduled maintenance to be carried out all the time. For your car I would recommend setting off £1500-2000 annually for this purpose.