On Oct 8, 6:29*pm, NT wrote:
Lots of people assume so. But its not how it actually works. Dual
circuits are no more reliable than single. The advantage is the
ability to vary the relative pressures to some extent as the back end
rises up.

Not since the 1980s! Those early dual circuits were there for
controlling brake balance to the rear, but they had no real
aspirations to reliablility as a safety measure. The first ones (my
1968 Fiat 124 AC) even had a rear brake proportioning valve, but only
a single circuit master.

A modern dual circuit, intended for robust fallback after a failure,
is diagonally split. If pressure to the rear-end is reduced according
to suspension travel is required (getting more rare with ubiquitous
ABS), then this is done by reducing the pressure in the rear portion
of both circuits. A heavier vehicle will also have duplicated front
circuits, where a caliper with multiple pistons has half of each
caliper supplied by each diagonal circuit separately.