i think it might be bubbles in the lines. when was the last time you bleed? it's pretty scary that if you press the brakes all the way and the car doesn't stop and you have to pump the brakes... i hope there is no leak anywhere...? check your reservoir, it's full?

 

Mine didn't go to this extent but I can foresee the pedal going to the floor.

If you first look at an exploded diagram of the GS's braking system, it mainly consists of (from the pedal)
- An integrated Master Brake Cylinder/ABS Unit
- Hard lines to the calipers
- Brake lines
- Caliper
- Brake pad

I literally upgraded one by one (starting from the pads) since I thought the initial pedal feel of the stock GS brakes were poop. Then somewhere along the line, I think my booster started to die...which didn't create enough pressure, and ABS freaked out and kicked in a lot under low mu roads.

So since I had replaced almost all the components except for the booster, I swapped the booster (since it is one assy) and I got back my pedal feel. If you can locate one, I highly recommend an 01-05 booster though (one connector will have to be modified however). I have heard that you can disassemble and fix the booster itself but I just replaced the entire unit.

Also, as noted in this thread don't forget to bench bleed the booster itself...which is PITA.