If you get flow to either of the two rear calipers then you should be fine in regards to the master cylinder. Somewhere going to the back is a tee where it splits off and goes to each side. Find it and loosen the inlet to the tee. If you are getting fluid to there then again, this should indicate a good master cylinder.

Another thing to try would be to loosen up the brake hose where the hard line ends at the wheel well. If you get flow then I would suspect the hose is bad.

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Jim