You need to adjust at the rear brakes - the further the drum-to- shoe clearance, the further the pedal has to travel before engaging firmly.
A good rule of thumb to use when adjusting the rear shoes- With rear tires off the ground, spin each one by hand. If should make 1 to 1/2 revolutions. Anymore than that, they are out of adjustment. If it doesnt make 1 complete revolution, they are over-adjusted.
Once this is done, if pedal is still soft, then it is time to do a brake line clamp to test in order to accurately identify where problem is occurring:
For a brake line clamp test, you need either 4 brake line clamps, or 4 pairs of vise grips(wrap the ends with cloth to prevent damage to brake hoses).
Place a clamp on each flexible brake hose about midway between the caliper and steel line. Do it at all 4 wheels for testing.
Now, test the pedal- should be nice and firm. If it drops to the floor, then the master cylinder is defective out of the box and needs to be replaced . If good, then start at RR and go to LR, RF, then LF- remove a clamp and test pedal. If good, move to next wheel. At whichever wheel the pedal drops, that is the wheel with the problem - kinked hose, damaged caliper that isnt pushing out...