Bad Master Cylinder or Air?

So I swapped my car from front drums to disc this winter. Bought a new master cylinder, swapped out the distribution block for a proportioning valve. Front calipers and hoses are new. The 1st pump on the pedal it almost goes to the floor but will stop the car but takes awhile. The 2nd pump has decent pedal and the 3rd pump has a really good pedal. Wait a few seconds and its back to the soft pedal. Initially I didn't bench bleed the master cylinder and left the distribution block on there. I have since swapped out the distribution block, bench bleed the master cylinder and continue to bleed the lines. I even left the bleeders open and left the fluid gravity feed for almost half a day (keeping an eye on the level and refilling as necessary). Even after this the pedal is still soft on the 1st pump. When bleeding the rear brakes the pedal only goes halfway to the floor (meaning the front brakes are functioning). But when I do the front brakes the pedal goes to the floor (So I'm guessing its the rear brakes that's the issue). Now when I did the gravity bleed, the front brakes reservoir I filled up twice, but the rear only once as it was alot slower to bleed even though I complete removed the bleeders. I'm going to try to bleed the rear brakes again, but is there anyway to determine if maybe my master cylinder just isn't working correctly? I've went thru 30-35oz of brake fluid in all of my processes thus far.