No brakes...

[GMC Guy]

hey guys, i'm having a problem with my brake system on my 79. started off with the pedal being super soft, randomly was fine one second, shut the truck off then boom soft pedal. soo, first thing i did was check fluid level, was low so topped it off. wasn't leaking brake fluid anywhere i could tell. that didn't change anything, so i figured i would change the master cylinder. it was old so i didn't have a problem installing a new one, even if that wasn't the problem. bench bled it for awhile even after bubbles stopped coming up. got it all installed and still super soft pedal, but only when the truck is running....soo i went ahead and changed the brake booster with a tuff stuff unit from summit. again, i had no problem spending money on it, even if it wasn't the culprit. i figure no money that goes into my truck is wasted, as i plan to never get rid of it.
the latest thing i did was bleed the brakes real good. i did noticed a weak flow from the driver side front caliper, not sure what that is trying to tell me.
that's where i left off, firm pedal with truck off. super soft with it running.. i've been waiting for my tax return to go ahead and do the rear disc brake swap and change all the rubber lines and flush the system real good. i'm hoping if i just replaced everything in the system that should fix it...right?