No Fluid Flowing During Rear-Right Brake Bleed

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Mazda3
I have a 2008 Mazda3 Sport, and am currently in the process of bleeding the brakes according to this guide posted by flatlander937.

I've followed the guide to a tee, with the sole exception of bleeding the rear-left/front-right circuit first, and then beginning with the rear-right as I bleed the second rear-right/front-left circuit. Bleeding the first circuit (RL-FR) was a breeze and everything went as expected - during the bleed process the pedal felt tight, brake fluid flowed easily through the bleed line, and the brake fluid level in the master cylinder reservoir drained as expected (with top-offs to ensure it didn't run dry, obviously).

However, when starting on the second circuit (RR-FL) with the rear-right wheel, no fluid seems to flow during the brake pedal pump process. There seems to be an unusual amount of air in the system, and despite 50+ brake pedal pumps, there is barely a trickle of fluid moving through the bleed line. I even tried using a vacuum pump to no avail; brake fluid just refuses to flow through the bleeder valve.

To troubleshoot I've replaced the bleeder valve itself, used a vacuum pump instead of manually pumping the brake, and even tried bleeding with the handbrake disengaged (it was engaged during the first circuit bleed). Nothing seems to make a difference, as barely any fluid flows through the bleeder valve, the brake fluid level in the reservoir remains constant, and the brake pedal is very spongy with no bite.

Where did I go wrong? What steps should I take from here? Is this a master cylinder issue? A blockage in the brake lines? Something to do with the ABS system? Brake booster failure? Why did it work for the first circuit?

Help would be appreciated.

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EDIT: I went ahead and decided to bleed the front-left wheel, ran into the same problem as with rear-right. So I guess that eliminates a blockage in the brakelines. Also went ahead and verified that the brake booster worked by pumping the brakes and starting the car. The pedal depressed as it should without much effort, so I believe the brake booster is functioning correctly. I assume this must be an issue with the master cylinder then? Or maybe there's some valve that is closed for this circuit?
 
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Did you see my comments in post #6? The master cylinder piston seal may be damaged.
 
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