SIMATIC

i think had enough. i just changed my calipers, n my master cylinder. The pedal feels better but...only when i press the pedal hard or stop fast, the pedaL SINKS DOWN(not to the ground but down enough.) My friend told me that the lines are not metal so when i press down on the pedal the lines expand giving the illusion that there is air in my lines. But my friends civic doesnt do that. Any 1 got any ideas or help?? thanks