WTF is wrong with VAG?!

[Nick M3]

So, due to clearance issues, I have been trying to retrofit an audi brake booster and master cylinder to the E30. This replaces that Porsche 924 booster that works, but doesn't provide enough travel for me to get into ABS. A little bit less than optimal with 321hp.

Fitment wise, this was easy. A little hard line bending and an adapter fitting and done. And, of course, a little bit of further modification of the intake manifold.

Function wise, it's been an unmitigated disaster. At least I've finally figured out why: It seems that if you bottom out the brake pedal, you ruin the master cylinder. Yes, that's right. Ruin it. Apparently the correct technique for brake bleeding is to put a piece of 2x4 under the brake pedal in order to avoid this. What in the actual fuck?

Edit: And to be clear, when I say ruin, I mean "not enough brake pressure to keep me from spinning a front wheel with one hand."

[JST]

How does it get ruined? What happens to it?

[bren]

I learned this lesson the hard way many years ago with a Honda.

[Nick M3]

Originally Posted by JST

How does it get ruined? What happens to it?

Apparently the seals invert, and it ends up doing literally nothing.

[rumatt]

Originally Posted by bren

I learned this lesson the hard way many years ago with a Honda.

The same is true for Honda's?

[bren]

Originally Posted by rumatt

The same is true for Honda's?

I don't remember spending much time diagnosing the exact cause of failure, but I've been careful not to push the pedal all the way down when bleeding brakes ever since.

...I just don't think the cylinders are designed with that much pedal travel in mind.