850Ci brake master cyl options

[LappingLuke]

Hey gang. I noticed my master cylinder leaking out the back this weekend. The BMW one for our cars is comically expensive so in looking around I read the e32 750 master is a valid option for a lower price. Is the part below a good choice?

https://www.fcpeuro.com/products/bmw...ro-34311156643

[DeltaSix]

I installed the ATE e32 750 master cylinder 6 years ago and still going strong.


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[shogun]

there was also some discussion here : Question-about-brake-master-cylinder https://www.bimmerforums.com/forum/s...aster-cylinder

from DIY section https://www.bimmerforums.com/forum/s...BMW-850i-(e31)

[LappingLuke]

Excellent thank you guys!

[Auraraptor]

Looks like the 840/M73 master cylinder is now NLA.

[cartoonz]

the M70 cars can use the E32 master
The M73 cars are a bit harder to deal with
Which are we referencing?

[shogun]

he refers to 840 and M73, in this thread was mentioned: ...I checked my 96 M73, and it’s definitely a three bolt master cylinder, which is the same as the 840 master cylinder.... https://www.bimmerforums.com/forum/s...aster-cylinder I found this: 34311162914 fits 840Ci M62 840i M60 850Ci M73

[LappingLuke]

For what it's worth, mine is an M70 car, which I originally intended to mention.

[cartoonz]

For what it's worth, mine is an M70 car, which I originally intended to mention.

As I thought... use one from a 1990 750iL. Works perfectly

[LappingLuke]

Is there a need to run any bleed procedure on the ABS unit after swapping the master cylinder?

[cartoonz]

Not usually, but it could happen

[LappingLuke]

Anything to look out for or avoid so I dont have to? If I have to, can my dragon850 laptop do it?

[cartoonz]

It could but you likely wont need to

[LappingLuke]

Okay good enough for me!

[Max Lumens]

I took physics for poets 50 years ago and forgot most of it, so someone relieve me of my ignorance. An e31 master cylinder is 1 1/16" diameter, an e32 MC is 1". For years, I figured that meant that the hydraulic pressure on the piston was reduced by about 6%, so you lost 6% of your braking capacity by using the 1" e32 MC. (1 divided by 1.0625)

But that's calculating by the diameter, not the surface area, of the pistons. When you use area, you see a loss of 11% using the e32 MC. (.7854 divided by .8866= .88585) Am I right?

[Timm]

Your mathematics are back to front - using an E32 Master Cylinder: for the same pressure on the brake pedal you will achieve a 6% increase in brake pressure - however the pedal will move further by 6%.

[Max Lumens]

6% or 11% ?

[Auraraptor]

Hydraulic pressure is force per unit area. So area is correct, not diameter.

Regardless as area changes for same force, overall pedal travel varies: More total piston surface area (think multi piston massive brake calipers), more pedal travel needed. The converse occurs for enlarging brake master cylinder size. Indeed, enlarging the master cylinder, is one approach to correcting long pedal travel after increasing the overall/cumulative piston area in the system.

So if you’re gonna do the opposite and go to a smaller master cylinder, you will be increasing travel as Timm outlined.

PS good to see around “Max”! Hope your doing well in the Pacific Northwest.
PS2: are you still in the import business? Shoot me a message some day, would love your input/services…

[LappingLuke]

It could but you likely wont need to

is this below what we're talking about, what I have referred to as "bleeding the ABS?" I am ready to bleed the brakes but the book is telling me I am not...

what exactly is "the mineral oil side?" I thought brake fluid was basically mineral oil. But so is some hydraulic fluid...

bleeding.JPG

[LappingLuke]

Considering this maybe I am okay to just do it the regular way.

bleeding2JPG.JPG

[cartoonz]

Just put a pressure bleeder on the reservior and bleed it... this isnt rocket science. Rarely do you have to do any ABS bleed procedure.

[LappingLuke]

Thanks as always. By now you and the other members are aware of my tendency to over think and triple check. You guys help more than you know...

[Auraraptor]

Looks like my M73 E31 master cylinder is now leaking.

Could one use a m70 Master cylinder/brake booster combo on an M73 car? Trying to figure out options

EDIT: nope, looks like they have a different bolting mechanism. :-/

[merlin840]

could you just not hone or re-sleeve and fit a new kit to these master cylinders?

i mean when i younger (30 yrs ago) that's what i did for a job if the kit isn't available there's plenty of other kits which you can use as it's not really a weird size like the 4 pot calipers on the 840 they use a 40mm piston and the other is a bigger common size which i can't remember from memory i ended up using a piston kit from an Italian tractor which did the job well which also reminds me the spline on the CSI clutch plate is the same as a Massey Ferguson tractor I'm sure there must be a company in the USA or Europe who must still do this the company that i worked for here in little New Zealand is still going strong and re-sleeving cylinders

[thelongeride]

I recently installed the e32 750 master cylinder. No perceptible difference in feel. I hope this helps, if you are trying to get opinions to help your decision.