Been struggling on getting my S4 turbo's brakes working properly.. When I push the pedal, it goes down more than usual then hits that 'fluid wall' where it wont go anymore. Last time I took it out for a test drive I had to push down with all my life force to stop. I've went through nearly everything and still same problem.
Heres what I've tried to no avail:
- Bled both brakes and mc 10000000 times. No air.
- Replaced rear calipers. Bled the secondary bleeders then bled lines again.
- Checked check valve. Works fine.
- Blew into the brake booster through the vac hole. At first there was air leaking from where the mc and bb connect, I sealed it with RTV. Held pressure both with pedal depressed and released. Could also hear the diaphragm expanding as it should toward the atmo side when blowing while pedal depressed. Tried sucking air out as well, still held pressure.

Very lost.. Brake booster seems mechanically fine, could it be the pushrod clearance? MC? Any info would help

 

Is the check valve installed in the correct orientation?

 

Yes, forgot to mention that.

 

we should note that the FC being from the distant past has a different brake feel than a new car, the FC pedal has a long travel and braking should be pretty linear with the travel, its not like a new car where they have no travel and you modulate with effort.

on mine the master to booster pushrod clearance was way off, its supposed to be around zero, and mine was like 0.030", since there is a 5:1 multiplier on that it makes a big difference

the other thing you might try is bleeding the fronts first and then the rears, apparently if you bleed the rears you don't have enough force to effectively bleed the fronts, if you've bled things a bunch probably not your issue.

a third thing might be brake pads, the factory pads have a lot of initial bite, and most replacements don't.

 

Do you happen to know if the MC was ever changed because it was leaking? At times when they start to fail the brake fluid makes it's way into the booster and starts wearing down the diaphragm. It's worth a shot. You just unbolt the MC from the booster and see if you see any corrosion and signs of brake fluid.

 

I have an S5 n/a still with the original brake booster. The booster is 32 years old with 360k+ miles. I think they're pretty reliable. Have you checked the MC? I go through these about once ever 7 or 8 years. Also, I'd recommend checking the front caliper pistons for free movement. I've had a couple pistons once seize and it took a lot more effort to stop the car like you were saying.