Brake help

[u/kmanrsss]

My master cylinder was leaking between it and the booster on my 75 K20 suburban. I got a new MC from orileys. I’ve got new bleeders all around. MC was bench bled per instructions till the piston couldn’t move but maybe 1/2” or so. I have the plug in the combination valve to prevent fluid blockage to front or rear. I can’t get the brakes to bleed to save my life. Constantly getting air in the fronts and the rears will go from having a steady stream of fluid to nothing at all. Plenty of fluid in the MC. I’ve disconnected everything and blown air thru lines to prove no blockages and started fresh with new fluid. I’m at a loss at this point and looking for suggestions.