I've been working on these brakes for weeks now.
1) Replaced the master cylinder, bench bled according to instructions, no air whatsoever coming through the tubing when I was done.
2) Replaced brake booster, old master cylinder was leaking back into it. The pedal wouldn't firm up with the engine off.
3) Completely flushed entire system. Old fluid looked like coffee.
I bled the system down again last night. With the car off, the pedal stays nice and firm if I tap it and very slowly goes to the floor if I stand on it, like it's supposed to. I thought the problem was solved...until I started the car.
The pedal's going at least halfway to the floor before it feels like there's enough pressure to stop the car. It's definitely better than before, but it just doesn't feel right.
Tomorrow I'm going to run a new vacuum hose from the plenum to the booster, invest in a Mity-Vac bleeding kit, and probably put new pads all around.
Can anyone think of any reason my brakes are doing this? The absolute only things I haven't replaced are the rubber lines, the calipers, and the proportioning valve

 

sounds like air still in the system
how are you bleeding the calipers?

 

The old fashioned way. Girlfriend works the pedal while I crack the screw.

 

id say the replace the hoses, im waiting on mine from hawks. i figure id work my way from the simple to the hard. I to suffer mushiness . replaced calipers and pads, shoes are alright but im getting to that .

 

When I did mine recently somebody suggested to tap the caliper while bleeding air likes to stick to the sides. Mc can be a pita you may still have some air in there. I would recheck everything again If you have high miles or just a old car replace the flex lines they tend to flex if realy old which and give you a softer pedal.

 

Mine are mushy too, replaced master cylinder, calipers, flex lines, rotors, booster, and bled the entire system. Then i took to brake check and they told me i had some leaking wheel cylinders in the drums in the back. After i replaced those it stops a lot better now, but still a little mushy. 3rd gen brakes blech.

 

I replaced both front calipers, pads, rotors, bled system. Brakes still mushy. I've mainly had all older cars and have learned to accept it. If I slam on the brakes they lock up and the car slides so I assume they are grabbing enough =P

However, getting out of my girlfriends 2001 Grand Am and into my 1991 Firebird always makes it feel like there is alot wrong with my car (Except of course for the handling!!) =(