Master cylinder/brake booster questions

[87Blue325iC]

Driving home the other day, my brake pedal went to the floor, brake fluid resivoir is empty. Just did ss lines and new brake fluid 8 months ago - fluid was correct last time I checked a few months ago.

No leaks that I can see - suspect the master cylinder, as does my neighbor the BMW dealer mechanic. He said the cylinder is probably leaking into the booster - hence to lack of missing brake fluid.

Sooo... should I upgrade to the E32 735/e34 M5 25mm master cylinder? Heard that the brake bias shifts to the rear, and I would need to use a remote resivoir. Is it worth it? Can I still use a stock booster? Should I just replace with stock parts. I'm looking for better pedal feel, but not if I have to modify a bunch of stuff. Yes - I've read all the old threads - still not sure what to do!

[2002maniac]

I would agree with your mechanic neighbor. The booster can hold almost a whole liter of brake fluid before you notice it being sucked into your motor.

IMO the 25mm master is too big for stock brakes. The M3 MC would be reasonable if you really feel like upgrading.

[Van Westervelt]

I do beleive however that the e30 m3 has different size lines going into it as most m3 guys have to change one of them to match up to the 25mm master, where as standard e30s will thread righ into the 25mm.

Meaning that the 25mm is plug and play, where as the e30 m3 one (23mm i believe) will require modification of one of the hard brake lines going into the MC.

[2002maniac]

hmm, I had a 23mm MC that I was told came from an M3 and it used the same size fittings. I have since swapped to the 25mm unit. (needed to push more fluid for my bug brreaks)

[Van Westervelt]

Hmm, I have talked with two m3 owners about it as I was thinking of using a 23 instead of a 25. One went to put in a 25 and it wouldnt fit, the other had to modify the connection to make it work.

This is heresay however, but I have heard it from more than just my two local sources. New 25mm's are only 120 or so anyway. And a lot of people run them on stock breaks, just depends on what you like.

What is this about the bias changing?

[2002maniac]

What is this about the bias changing?

The stock MC has a stepped bore with less fluid going to the rear. The 750MC is 25mm front and back.

[87Blue325iC]

Ok - I went with the stock brake booster and the 25mm master...do I, or do I not, need to relocate the brake fluid reservoir? Motor and airbox are stock.

[Teaguer]

No need to relocate anything .
The 750 m/c is a direct replacement .

Would not worry about it being biased to the rear either , this m/c swap is a common practice for auto-x and track cars across the country .
I run 3 different cars, two of them with stock brakes, with this set-up and it makes a huge difference in pedal feel but no adverse effects on the biasing .

About the M3 m/c needing to change a line is correct , the ft threaded part is different size on the m3's .

[87Blue325iC]

No need to relocate anything .
The 750 m/c is a direct replacement .

Would not worry about it being biased to the rear either , this m/c swap is a common practice for auto-x and track cars across the country .
I run 3 different cars, two of them with stock brakes, with this set-up and it makes a huge difference in pedal feel but no adverse effects on the biasing .

About the M3 m/c needing to change a line is correct , the ft threaded part is different size on the m3's .

Thanks!

[87Blue325iC]

Next problem - missed ordering the O-ring part#34 31 4 650 345! Any suggestion on where to get one fast? I'll try the dealer, but doubt they stock too many E30 parts...

[Teaguer]

Your old one might just transfer over .

But if not most real shops carry a bin full of different o-ring sizes .
Take the 750 m/c with you and just size it up .