Chevy Blazer Brake Problem

I have a 95 Chevy S-10 Blazer with 4WD. When I initially come to a stop the pedal is nice and firm but after I'm stopped in traffic, the
brake pedal will slowly sink to the floor. I checked for external leaks and found none. The master cylinder has been replaced 4 times, the last time with a new (not reman) unit. The BPMV for the anti-lock system has also been replaced. But the problem remains. Are combination valves known to leak internally? Is there air in the system or BPMV? Can the BPMV be bleed without a scan tool? Bleeding the system manually does not show air and does not correct the problem. Any other ideas? Thanks in advance!
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