[sideshow]

get 4 vise grips

clamp off all 4 rubber brake lines that goto the wheels
then press the pedal it should be rock solid
if it is not rock solid then u have air in yr master cylinder or faulty master cylinder

if it is rock solid then unclamp each vise grip one at a time
and after each one test pedal again
the wheel that softens the pedal the most when the others are clamped up is the problem one