Disc brakes feel spongy no matter how much I bleed them

Need to be clear on the symptom.
If the peddle stops at some point and does not bleed down, that tells me the system is not leaking.
If it is "spongy" I take that to mean it does not have a solid feel but it does stop at some point.
If this is the case, I would suspect air in the line as the most LIKELY cause.

Do you have an air compressor?