There is NO engine vacuum brake booster on a diesel engine because there is no Engine Vacuum on s Diesel, as strange as that may sound. I used to be a Ford Journeyman and I have worked an lots and lots of Ford Diesels. Diesels need a separate vacuum pump to power things like the heater controls and the brake booster.
I would look for a very small leak or seepage. How is the area below your master cylinder, if the paint is blistered, then you have located your problem. Have you removed the read drums and looked at all of your junctions and the metering valve/proportioning valve?
Have you looked inside your Brake Booster Diaphragm? If it is wet, then the Booster is leaking.