I had an '85 W150, and the brakes kept getting air, finally figured out the exhaust was blowing on one of the rear brake lines on the diff. The PO had run a new line and had looped it above the housing, getting the heat from the exhaust. Ran a new line properly, and that solved that.
and second the the master cylinder being suspect...just becaue it's new doesn't mean it's good now a days.