Try rapidly pumping the pedal rapidly, 3-4 times hard, and then immediately push again and hold and see if then the pedal goes hard before reaching the floor. If this happens, it is a sure sign of air in the system.
The other area of softness is in the rubber lines. If they are old and cracked inside, they will tend to expand with pressure. Go to stainless braided covered teflon hoses; the will make firm brakes rock hard, and give great pedal modulation.
