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The check valve mentioned actually is located underneath the tube seat(s) for the lines coming out of the MC, you cannot see them by removing the bowl cover.The disc brake MC is supposed to have a check valve in the line going to rear brake, while drum brake MC has check valves in both lines.I really doubt your problem is there especially if they worked OK at one time.
Since brakes are the topic, I hope I'm not straying too far..... But has anyone had any experience with Apple Hydraulics rebuilding their calipers? It would be the closest place for us on the east coast. White post seems to have marginal reviews.