First I would screw block off plugs in the master cylinder where the lines go. The pedal should be "locked" if mc is good. Next try reverse bleeding since your wheel cylinders are higher than the mc; put a catch pan under the mc. 20psi is enough to over power any residual pressure valves. Reverse bleeding can be messy so we use an old Wagner bleeder ball where a diaphragm pushes down on the fluid in the ball and forces a pressurized all fluid,no air, stream through the lines. With a two gallon capacity one can leave a bleeder valve open till the stream is completely clear. I don't know if one can rent these but if you can I'd nearly bet the problem will disappear.

Evan