# Help me understand the brake system..



## NewBmw733IDigIt (Feb 25, 2009)

Okay..I am new and I am learning as I go. BUt what I have learned is this..The brakes and the power steering is intertwined in some way..

WHat I have noticed is that for some reason I am losing brake fluid..When I fill the resevoir it goes down..And goes down, and goes down..Then I refill it and the level drops more..I see nothing leaking out..I even put cardboard undereach wheel and under the master cylinder..Am I filling this thing up incorrectly? 

It is a 1982 733i with power assited steering..

Any help will be much appreciated..

Thanks everyone..

Have a great day!!


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## crewdog843 (Mar 15, 2006)

Hopefully you are pouring the brake fluid into the correct reservoir.

jake


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## NewBmw733IDigIt (Feb 25, 2009)

*Jake...Yea..I am sure of that!!!lol*

Jake.

I got the right spot..And I learned that the power steering and brake system is somewhat connected. In the manual says if you take the powersteering fluid cap off and pump the brakes ten times or so It will raise the level in the power steering res.

When I do this I notice a major dif in the brake pedal. I bleed the brakes and It is fine for a few days. Then the brake light comes on and the level is way way low..I have checked the whole system from front to back. There is nothing leaking. No drip marks no fluid anywhere..I am so stumped..I checked the master cyclinder..The is zero pedal fade. I am just lost..I know the has got to be something I just have no clue where to start...

Any real help would be most appreciated..

Thanks

Daniel


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## joegwillett (May 3, 2009)

Does the 745i share the same resevoir for the powere sterring and the power brakes?


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## mosearch (Oct 4, 2006)

joegwillett said:


> Does the 745i share the same resevoir for the powere sterring and the power brakes?


I have never heard of a combined system, and find it hard to imagine how or why it would be done that way...still I see why you ask, given the symptoms described.

Here's another odd question: Since the there is a lot of brake fluid going in, and apparently none coming out, IS THE POWER STEERING FLUID level rising?


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## drivinfaster (Nov 24, 2007)

i keep spending waaay to much time in ot, or i would have posted earlier.

the 7's have used a hydro-assist system on many models of the 7. the same pump that provides assist for the brakes also provides assist to the steering. the fluid is specific, and should only be what is listed. do not use regular power steering fluid or automatic trans fluid on the pentosin systems. (this should be listed on the cap of the reservior) do not confuse brake fluid with the power steering fluid. brake fluid is not compatable with any other fluids. here's a pic to help, the metal cylinder with the white cap (the one that looks like it has triangles on it) is where the power steering fluid will go. (or pentosin, if required on some models) while the brake fluid goes onto the plastic reservior (that you cannot see) which is located closer to the firewall under the coolant reservior. 
http://autospeed.com/cms/gallery/article.html?a=2031&i=4

here is a link to what the 'booster' looks like. it is not a master cylinder, but attatches to it. 
http://www.pelicanparts.com/cgi-bin...2-825-M4&catalog_description=BRAKE CYLINDER *

it should be under the coolant reservior, between the firewall and master cylinder, on left hand drive models. this one doesn't appear to be too difficult to do, however, from experience, i will suggest that you also order a new brake switch. plastic gets brittle, and most parts houses will not have one on hand.

in order to check this unit for proper function, i suggest using pinch offs or needle nose locking pliers with rubber tubing placed over the jaws to protect the rubber brake lines. pinch off the rubber brake lines and with the vehicle running, depress the brake pedal. (you will need to pinch each wheel off at the wheel on 4 wheel discs by the way...)

the pedal should be rock hard. if it bleeds down then the seals around the piston applying force are allowing brake fluid to pass. even contaminated fluid (such as hygroscopicly contaminated fluid) will not compress. rule of physics, fluids do not compress.

if the pedal is rock hard, then 1 at a time remove the clamps and see which makes it go soft. this will help you pinpoint what component(s) are leading to loss of fluid.

my guess is the simple money is going to be on the assist unit. if you do replace this, then i suggest upgrading to a 750 master cylinder, #34-31-1-156-643-m4. i could not find a listing in pelican parts for your chassis, but they all pretty much have the same fittings and locations. the 750 mc is a populat swap for e30 chassis to improve overall braking due to a larger diameter bore of the mc. (21mm i think, but not 100%)

you will most likely find lots of fluid in the cavity of the booster unit, so be ready for a mess. you may also need to adjust the rod that attatches to the brake pedal as well, so be prepared to break that locknut free before you remove it or else you will need to place it in a vice once it's out.

other than that, i think it's a pretty easy job. i did my 750, and it was tight. the inline 6 is more forgiving with room. just be careful with the fluids on the paint. this will be a mess job. (or did i say that already??)

good luck. feel free to pm me with any questions. i will try to be helpful.

df


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