How do I isolate the leak?

Sorry bad explanation. I guess a more accurate description is that the pedal is "springy". I can drive around my neighborhood and it'll stop if I push the pedal to the floor, however, I need to use a lot more muscle to get it to work. Plenty of brake fluid in the mc no leaks.

I don't quoted understand why I should back up and slam in the brakes...? What will this do? I'll do it I just want to understand what I'm doing.

I bled the brakes with the brake bleeder kit that creates pressure on the mc and you can open up the bleeder valves with one person. I bled the rear passenger drum them the rear drivers then the front pass her and the. The front drivers side. I opened up the valve and watched for bubbles and when it was clear I closed it and moved to the next one.

Thanks.