A good way to narrow things down in this situation is to check and verify the various components as much as possible before tearing things apart.
To verify the booster is working correctly, hop in the car, and with the engine off, pump the brake pedal ten times to use up the residual vacuum.
The pedal will seemingly rise up and get harder to push as the vacuum is used up and "assist" is lost from the booster.
Now, hold the brake pedal down and light up the car.
The pedal should drop noticeably.
This quick test will indicate the booster is functioning correctly.
If it passes this test, you can verify that the master isn;t bypassing internally by just holding pressure on the brake pedal for a minute. If the pedal drops, fluid is bypassing around the cups in the master and it's probably junk.
If the pedal doesn;t drop, then it's time to look at the other components of the system.
Air in the system will give a spoongy feel.
Had any part of the hydraulic system been opened, or components replaced before the problem appeared or did you buy the car in this condition?
[edit] I see that you bled the system, are you sure you got all the air out?
Did you bleed the master?
Did you bleed the abs valve body?
/edit
A siezed slider/pin on a caliper can also give the pedal a spoongy feel.
Does the car pull/feel weird under hard braking?
Are any of the wheels significantly hotter/cooler than the rest?
... bear in mind the rears will be somewhat cooler than the fronts, but they should be approximately the same side to side, ... do NOT touch the rotors, (that sh*t is HOT!!! ) ... just feel the wheels.
At this point, you have to start somewhere, so please, check the booster and master cylinder as outilned above and update the thread so we can advise you further.