Hi Simon,
Initially, i.e. while the fluid is taking up the clearances in the calipers
and drums, how much each piston moves is governed by the path of least
resistance as someone has already said. If the secondary circuit has more
resistance then the primary circuit will 'fill' first, and the spring between
the two pistons will be compressed. When the primary circuit is full all
further movement of the primary piston will cause an equal movement of the
secondary piston to fill the secondary circuit. If the primary circuit has
more resistance then the fluid between the pistons will cause the primary
piston to push on the secondary piston and move that an equal amount
compressing its spring (where fitted) first. If the circuits have equal
resistance then both springs will tend to compress equally, and the secondary
piston will move half the distance of the primary piston. But all that is
only when taking up the clearances. The springs are mainly there to push
both pistons back when the pedal is released, a stop controlling how far back
the secondary piston can move. It's the later dual master with integral
balance switch that has dual springs, the earlier dual master that used the
remote balance switch only has one spring between the pistons, its is the
primary piston returning that pulls the secondary piston back.
You might also be interested in this with animated diagrams:
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake1.htm
PaulH.
----- Original Message -----
Presumably, the way it works is that when the pedal is initially
pressed, the spring causes the secondary cylinder to move until the
primary cylinder has moved enough to begin to raise the pressure in
the primary circuit...
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