I worked at a place where we rebuilt these type units. Had a customer with a 1934 Cadillac gives me a very similar unit to do. BK is an early name for Bendix brake. (The Cad brakes were very similar to the Chevy style, just larger.( ie S-cam, Huck style). This unit only uses vacuum to "assist" you with brake pedal pressure. Also similar to the vacuum assist for shifting that came out in '39. I was very interested in my customers reaction to the "assist" he got after the unit was put back into operating condition. He said, "I can feel it working now, but really doesn't do a whole lot." Of course, this unit is a precursor to the Bendix Hydrovac which used vacuum and hydraulics to make braking easier in cars and trucks.
It might be interesting to install one in your car and see what it does to "assist". Don't think it could hurt anything.
Of course, high quality wireback or HD open woven lining develop the most friction, with the least pedal effort ,and will give you the overall best braking results, with or without an assisting unit.
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