Once you have bled the air from the brake lines,clamp off the front brake hoses carefully.Does the pedal feel better now?
Then the pad /caliper fit is allowing too much play.Is the pedal the same?Then clamp the rear hose and try the pedal.Is it fine with the rear hose clamped?If so we now know the problem is at the rear brakes.One common low pedal rear brake cause is brake shoes that do not fit the drums.Remove the drums and look at the shoes,are they showing contact wear fully or just in the middle of the shoe?Remove a shoe and place it in the drum.Can you rock the shoe against the drum surface?Once drums have been resurfaced,the shoes will not fit fully against the drum allowing the shoe the flex when applying pressure to it and this can easily create a low soft brake pedal. Of course rear brake adjustment must be correct once brake shoe contact is correct.To correct brake shoe contact,have your shoes re arc-ed to fit resurfaced drums or install new drums.Don't overlook brake master cyl /brake pedal push rod adjustment too.