I was surprised that the discussion of the differences between
K and AB brakes did not also point out that the AB's mechanical
design and construction was vastly superior to the K brake.
The K brake consisted of several sliding brass plates with holes
in them, which, depending on their position in the body would open
or close certain air passages. These sliding surfaces were
notorious for becoming non-functional for a number of reasons,
including sticking due to contamination by grit or frozen
condensation, and leaking due to wear.
By contrast, the AB brake works by opening/closing orifices
with rubber diaphragms. No sliding motion to get stuck or leak; far
more tolerant of moderate dirt or water contamination; and needs
no lubrication.
Thus, in addition to the as-designed performance improvement
of AB over K discussed in previous posts, they also kept on
perfoming as designed to a much better degree and were more
durable and required far less maintenence.
All very compelling, which explains the urgency with which the
AAR wanted to achieve the conversion even in the depths of the
Depression.

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Jeff English Troy, New York
Proto:64 Classic Era Railroad Modeling
englij@...

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