Residual pressure valve of front disk brakes.

Anyone ever use a residual pressure valve on the front disk brakes? If so
what pressure did you go with? For those who haven't heard of one of these
before... They're a pressure valve that keeps a couple to a few pounds of
peddle or brake pressure in the line (normally the front) to maximize and
quicken your braking... or at least that's how I understand they work.
Thanks a heap,
-Drew in San Jose