I was pretty sure that the 'serial' systems incorporated two pistons mounted on
a common pushrod and are therefore equivalent to two 'parallel' systems.
The thing to recognize is that the brake master piston area is the sum of the
two individual pistons. Let's say that an original system used a single 13/16"
master cylinder piston, and that 300 psi line pressure was required to produce
incipient lockup. So then the pushrod force is 300 psi * 0.518 square in. = 155
lb. With a pedal ratio of 5:1 that works out to about 30 lb at the pedal -
pretty light. So my numbers are wrong, but the math will work out.
Now assume a dual master system was installed with identical 5/8" bores. Since
it will still take 300 psi *in each system* to produce incipient lockup
(disregarding brake balance problems), the pushrod force required is 300 *
0.3066 + 300 * 0.3066 = 184 lbs. Strictly speaking, the pushrod force is the sum
of (line pressure * master piston area) for all master pistons. Using the same
5:1 pedal ratio gives about 37 lb pedal pressure. Note that due to the increased
master piston area, the pedal travel required to take up mechanical slack in the
pads and shoes has been proportionately reduced. The pedal will feel firmer, as
well as requiring more force to stop the car.
The line routing does not affect the braking force required one iota since the
brake fluid does not 'flow' in the system. The only thing that will affect pedal
firmness is flex in the calipers and rotor assemblies, and rubber line stretch.
Theo Smit
tsmit@novatel.ca
B382002705
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