You can test the master. With line clamps, or padded vise grips (use corrugated carboard between the jaws and the hoses) clamp the three rubber hoses. Try the pedal. If it is rock hard, the master is OK, if still soft, replace/rebuild the master. IIRC, a rebuilt master is less than $100.
If the pedal is hard, remove one clamp at a time. If the wheel the hose connects to is good, there will be a small drop but still a hard pedal. Replace the clamp and go to the next one. Repeat. You will find the bad wheel this way.