thats exactly what happend with me. we put new suspension on my step bros s2 along with calipers. were gettin fluid through but just a crap pedal. we went thru a hole lot of fluid 2. although messy, block off all outlets of the master cylinder and see if the pedals any good then.obviosly this is the last thing to check but if u say uv bled and bled it then its the only thing to do.
mk3 astras are always doing it, when u bleed them its advisable 2 just push half way down when bleeding.
if you completely drind the system then id say almost certainly the master cyl.