The original brake master cylinder on the E30 M3 has differently sized holes for the two
brake pipes which run up to the ABS pump. One hole is sized to accept a standard 10 mm
fitting but the other hole is larger at 12 mm. Why this was done is unclear, there is no
advantage to it. In any case, the bigger 25 mm master cylinder has two 10 mm holes so this means
that one of the brake hard lines has to be replaced with one which has 10 mm fittings on
both ends instead of a 10 mm and a 12 mm fitting (all lines connecting to the ABS pump
employ 10 mm fittings).
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New brake lines are available from the dealer at around $10 each for the length of interest here.
Unfortunately, these new lines no longer come pre-bent. They come straight as shown in the picture.
It is left up to the installer to bend them to fit. The proper way to accomplish this is with a small
diameter tube bender. But it is not absolutely necessary to have a tube bender.
I bent the brake line using two big sets of pliers
with duct tape wrapped over the jaws to prevent them digging into the line. This worked quite well.
The BMW lines are thick so there is no danger of kinking the tube. But you cannot straighten the lines
once they are bent. You can "unbend" them a bit to fine tune their routing if need be, but once they are bent
you are committed. I was not able to perfectly match the original routing of the factory brake line.
But I got it close enough as shown below.
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The green circles show where the new line (with its 10 mm fittings) connects to the master cylinder and
the ABS pump. You can also see the routing of the line which is good, but not perfect. I covered the
brake line with rubber fuel hose wherever it contacted other brake lines or the chassis. This is to prevent
the line from being damaged due to long term vibration. Part numbers for two possible brake lines to use when
installing the 25 mm brake master cylinder are as follows:
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p/n
34 32 1 153 688
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(length = 590 mm, I used this)
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p/n
34 32 1 153 689
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(length = 620 mm, may have changed to p/n 34 32 6 755 585)
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There are other length brake lines available at the dealer. The original line is 603 mm long. The line
I used is a little short but it works fine. I have no trouble with the install and have auto-crossed and
tracked the car with the new master cylinder. It does indeed give a very stiff pedal. This mod is highly
recommended.
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