Ok, so in light of the discussion about pedal travel, I've begun to look at the brake servo as a possible problem area.
While putting together part numbers for price estimates, I discovered a check valve on the servo (part number 191.611.933.F for those liturature-abled).

What is the function of this part?

What happens when it fails?

I don't want to buy a booster if this may be the problem area...

 

This vlave holds the vacuum created by the closed throttle in the brake booster, it is simply a one-way valve, it also stops the boost being applied to the brake booster, to test just unplug it from the brake booster after the engine is stopped, you should hear a rush of air, if you don't either the diaphragm in the booster is leaking or the valve is shot. Test the valve by blowing air through it, you should only be able to blow in the direction of the inlet manifold.