Brake Improvements - Looking for thoughts

I just went out and measured (well attempted to) pressures.

Front left gave me 700psi under what I consider a normal pedal effort and 1400 under a panic stop
Front right gave me 800 and 1500, so those make sense.

I went to do the rear, removed the bleed screw, put my gauge in, and have zero brakes, pedal right to the floor.
I put the bleeder back in and left it cracked and pumped the brakes a half dozen times, nothing came out, tightened the bleeder back down and still nothing, right to the floor.

That tells me that I have a ton of air in the rear portion of the system, but so strange that the front could do 1500psi with a firm pedal, and then nothing. Or something is up with the master, as the pedal was rock solid when I checked the front pressures.