"When I was bleeding the rear calipers with the Mityvac, at first there was lot of bubbles coming into the jar. Later, at one point I was barely getting anything, then I increased the vacuum to about 25, and slowly but surely the fluid would start moving again, then I got a big burst of bubbles, and then it resumed a normal flow. It seemed like there was blockage that moved as the vacuum increased."

When vacuum bleeding (as opposed to pressure bleding) it is common for air to come in around the threads of the bleeder valve.
While benign, it can be difficult to distinquish between air in the line, and air that comes in around the valve. The best solution I have found is to remove the bleeder valve, and apply teflon tape on the threads.

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1986 300E 5-Speed 240k mi.