How would one determine if the master cylinder went bad or it's air in the brake lines?

Swapped front brakes and rotors but can't get the brake lines to bleed. The peddle felt like it was getting softer over the past week prior to changing and ABS light turned on. Basic mechanical abilities and contemplating taking to the shop.

Thanks for any input.

You tried the standard procedure and couldn't bleed the brake lines? I copied it from the service manual.

We did everything except for attaching the bleeder hose and fluid container. I was running the pedal and according to my helper(father) we aren't getting any fluid coming out of the bleeder. I'll give it another go this weekend. I just hate to start replacing things that aren't needed.

Thanks for help.

It turned out the left rear caliper was leaking allowing air in. We got it sealed up and everything worked as expected.