For no apparent reason my rear brake circuit disengages at times. I had the entire master cylinder replaced including the ABS (the truck has rear ABS only). I had the problem before the replacement; it was better after; now after a few hundred miles it seems to be getting worse. Twice now the damn thing gave out without warning almost causing an accident. I don't hear the abs pump at all whenever it decides to let go. Any ideas? The mechanic is scratching his head.