Pressure bleeding is the only way to go. Start with the longest line first (right rear). It your brake system is sound you should complete the job in 2 passes, 3 at the most. If that doesn't work start looking for a problem, like a leaking wheel cylinder, or cracked brake line.
Soft pedal means air is getting in somewhere and until you find the source you just wasting brake fluid trying to bleed the system.
I strongly recommend anyone getting a new car to do a full brake overhaul, including wheel cylinders, lines, shoes, master and turning the drums. If you don't drive the car a lot switch to Dot 5 fluid. Do that at the start you will have years of trouble free and safe brake operation.