This is the dual master cylinder setup that Jesse Ybarra is using on his 4-wheel
disc brake, M-679 Van. It is running the large 88 Suburban K-30 front calipers,
2004 Chevy 1 ton rear calipers, and he is using two 1 1/8” diameter master
cylinders from Willwood Industries. The mounting pieces are 2 stock FC brake
pedal pieces with an additional upper bracket and a thick rear mounting plate
that is bolted to the original bolt holes in the body, the pedal mounts are
bolted to the large thick plate, to combine the two brake master cylinders, the
left pedal is stock, the right pedal is stock but reshaped to bolt to the left
pedal. (this is an automatic, but the stock clutch pedal and mount can still be
designed for use with this arrangement)
Jesse uses two separate cylinders to provide the individual adjustment of different strokes for the two separate front and rear brake systems, while still using only one combined brake pedal. A larger diameter master cylinder, is needed for the larger displacement caliper pistons, this system requires less stroke depth movement to supply the calipers with enough pressure to easily, but sufficiently slow down and stop the vehicle.
Different from a longer length, single dual master cylinder system, that uses a dual outlet, inline master cylinder, that has 1 shaft, that internally pushes, 2 different pistons at the same time at the same pedal depth, these have no individual stroke depth adjustment and share the same stroke foot pressure, these then have a dependent requirement of an additional equalizer and mechanical pressure regulating adjustable pressure valve compensation. What that all simply means is that if the rear calipers are stopping sooner than the front, there is no separate stroke adjustment in the single master cylinder system and other external valves are required.
Additionally with two different master cylinders, these are two completely different separate braking systems, each can put out an easy 1200 psi, with the 2 different master cylinders, the stroke requirement may be individually adjusted to change the depth of travel of each of the separate pistons by changing the shaft or rod nut adjustment.
Another advantage of dual systems, is if this was a disc front / drum rear, then the proper calculations can be done to match the correct master cylinders for front and rear, for example a 1 1/8" master cylinder could be used for the front and a smaller 7/8" master cylinder would be used for the rear drums.
But the best thing is, if a line is ruptured or torn out, the separate system is unaffected.