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I'm looking for a master cylinder for a e31 840ci (pn#34311162914). If anyone has any info on swapping it for the 850-master cylinder and booster, please let me know. VIN# WBAEF8327VCC31287
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As far as I was able to find this winter when it concerned me, the 850 brake booster 34331161772, is not available anywhere. I had to send mine to Jay's BMW parts (https://www.jaysbmwparts.com/work/brake-boosters) to be rebuilt which was done quickly and for a reasonable price and I have no complaints. In fact I found him to be a good resource who replied quickly to inquiries.
Your only option is probably to rob a booster off a parts car and have it rebuilt. On top of that, you must make the minor sacrifice of using the E32 master cylinder which is slightly smaller piston. You'd expect to experience a longer brake pedal but I can tell you my brake pedal feels great now that the booster is rebuilt, accumulator replaced with new and new master cyl so no real sacrifice there. If you wanted to use the original master cylinder for 850, you can do it but it literally costs $1800 vs the very affordable E32 MC (100 something bucks I think on rockauto).
Beyond that, if the fittings are the same on the booster and the guts are all the same between v8 and v12... I have no idea. They look similar but I see there is some kinda adapter/dropbox thing that bolts to the V8 booster and then to the firewall. If that dramatically changes the position of the fluid IN port, you might be screwed because that is a hard line and you are not going to relocate that easily.
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The brake booster internals are similar between the 850 and the 840 and can be easily rebuilt. Now the MC is different and specific for 840 and M73 850, I am starting to work on a solution for this.
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