Brake Master Cylinder Questions...

[clarko74]

I'm looking at fitting a larger bore brake master cylinder (as I'm fitting bigger brake calipers) and have sourced a 23mm diameter one from an Audi that should fit.




The Audi m/c however has 4 connections for brake pipes whereas the e30 one only has two. What are these additional connections for and am I ok to blank them off and just use two?

I'll obviously use one output from the front and one from the rear of the m/c so the front and rear brakes remain on different circuits but apart from that does it matter which ones I use?

And one other thing, as I am yet to remove my original m/c and reservoir: does anyone know the dimension between the holes on a standard e30 reservoir? The dimension is 90mm on the Audi one and I'm wondering if the e30 resevoir will fit straight on?

[jmc330i]

Will a 23mm m/c be enough of a jump from the standard m/c? Most people go for a 25mm m/c when changing to larger calipers.

Yup, just blank off the two connections you don't use - you can buy brake union blanks for this purpose (basically a brake pipe union without the hole in the middle).

My Audi m/c looked different to that one (was from an Audi 90 Quattro iirc). I had to use a VAG reservoir as the reservoir ports were about 25mm apart.

[clarko74]

I hope so! I've got it now so I might as well give it a try.

The one I've got is from an Audi 5000. The increase in area over the standard m/c is still quite substantial. Here's what I have calculated:

• Standard - Diameter: 19mm - Area: 284mm²
• Audi 5000 - Diameter: 23mm - Area: 415mm² (46% increase in area over standard)
• E32 750i - Diameter: 25mm - Area: 491mm² (73% increase in area over standard, 18% increase over 23mm m/c)

I'll report back on if it's any good when it's fitted.

[DanThe]

If this is for the 540 calipers then you will require less pedal effort but the pedal may be slightly longer then the 540 master cylinder

Well worth trying out.

[HairyScreech]

Another option to consider if the pipe unions are a funny size is to fill them with braze or have someone weld the hole up. Both will be more than the fail strength of the pipe flairs.

[Lurch320i]

I didn't use the 750i mc when I upgraded to 540 brakes
I used the e32 735i master cylinder and servo combo

That too it 25mm