I've done complete brakes all around. I've checked the vac pump and the resvoir is air tight. No leaks and completely air free in lines. Also replaced rubber brake lines with braided lines. The pressure diff switch plunger activates when pedal pressed. I've got Disc in rear and there adjusted right. I've got sediment in Master cylinder reservoir after several good flushes with synthetic brake fluid. Could the master cylinder be failing and I'm getting junk from my MC back thru my check valves? I've got 130,000 miles onthe truck and it's the original MC. 

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