Pedal travel

Hello i have a e30 325 with ap racing 4 pistons (1,5 in and 1,62) in front and wilwood 4 pistons (1,75) in th rear, i already know that i have a big calipers in the rear. I got a brake booster from a porsche 944 and a mc from a bmw e32 750i.

My problem is when the car is turn off i have an hard pedal but when i turn the engine on it becames soft and make a to much travel, i even try to delete the front brakes only for test and the is hard like a rock but when i turn on the engine it becames soft.

Hey Pedro,

Lots can factor into this.

Do you know the diameter of the master cylinder? I did some quick research online, it looks like maybe 25mm? (probably not a bad size for your calliper size)

Sounds like maybe the booster is the problem. Seems over-boosted to me.

Depending on your rotor and pad setup, you've likely got a very rear-biased setup with that calliper combo - but it seems like you know that.

If it's a simple case of turning the engine off and the pedal instantly getting hard, you may have a faulty or missing check valve - you should be able to use it two or three times before getting really hard.

The pedal getting softer and a small increase in travel - because more line pressure is developed - is normal, but it sounds like you have way more than that. Normally I'd be thinking you need to re-bleed the brakes, but it could be the increased line pressure, for the same pedal force, is also causing caliper flex, hoses to swell a little more, or other issues.

What process are you using to bleed the brakes, as some manufactures have specific recommendations?

I think the problem is over-boosted because if just put the foot above the pedal without making force it already loock the wheels, so my next question is can i resolve that without swaping the booster? Or is same why to test it?

Ah, I suspect, from that last bit, there may also be an issue with the assembly - the pre-load/free clearance needed to have the system work as a whole.