Weak Brakes - Master Cylinder?

[aerosam]

Hi guys - had my car out on track yesterday, and the brakes were pretty lacklustre. It's all working correctly, stops in a straight line,they're just a bit weak, I really have to stand hard on the pedal.

They are sierra fronts and BMW rears (discs). The rears are huge so power is re-directed forward by a bias control valve. The master cylinder is out of my donor BMW E34 5 series, and it had a hydraulic powered servo, which I didn't use as it was very long and the hydraulics were complex.

So, I'm guessing the unassisted master cylinder simply can't provide enough stopping force. Isn't the roadster normally fitted with a sierra master cylinder without the servo? Is there room for the servo, if required?

I'm thinking of replacing my donor cylinder with a sierra one, with servo, in order to seriously upgrade my braking power. Unless you guys can think of anything else that would be hold the brakes back?

[michael92]

By no means am i braking expert but just by reading what you have said i too would assume its the reason of a non-servo system, i also assume that there is no "spongy-ness" ?

[MarkB]

You don't want or need a servo on such a light car, what pads are you using as normal road pads won't take much punishment on a track day they will fade!!!

[davidimurray]

What is the diameter of the E34 master cylinder compared to the sierra one? Have you checked the hydraulic ratio compared to the original sierra setup?

[aerosam]

To answer all 3 questions:

1. No there's no spongeyness, brakes all bled nicely.
2. EBC Greenstuff pads all round
3. Don't know the dia of the ATE master cylinder - still trying to discover that. I' slightly embarassed to say I don't know what you mean by hydraulic ratio - could you explain please? Thanks!

[aerosam]

Ok - after lots of digging through BMW websites, I've found the master cylinder is 25mm diameter.

[allterrain]

Hi aerosam, there are a number of articles on the web regarding mcyl sizing and pedal ratios which should help, basicaly a smaller diameter mcyl will give you more brake pressure with slightly more pedal travel and you shold have a pedal ratio of about 6 to 1. Here are a couple of interesting articals; http://www.hotrodheaven.com/tech/brakes/ and http://www.markwilliams.com/braketech.aspx . Cheers Brendan