The pressure *does* equalise in the MGB master cylinder. The pedal pushes on
the first stage of the piston, that pushes on the fluid for the primary
circuit (front brakes) which also pushes on the second stage of the piston,
which pushes on the fluid of the secondary circuit (rear brakes). There is a
spring keeping the two sections of the piston apart. It is not until both
circuits have taken up all the free play that either circuit starts developing
any pressure, which means that the pressure in both circuits is equal - under
normal circumstances.
With a failure in the primary circuit the primary piston cannot develop any
pressure in the fluid to start moving the secondary piston, until the primary
piston has moved far enough forward to start moving the second piston
mechanically rather than hydraulically. The secondary circuit is then
pressurised, but you only have about 10% braking as that is for the rear
brakes, and the pedal has moved further downwards.
If the secondary circuit fails the primary piston moves the secondary piston
as normal, but that cannot develop any pressure until it reaches the end of
the master cylinder, and it is only then that pressure is developed in the
primary circuit (front brakes). This also results in a longer than normal
pedal movement, but only 10% less braking effort. Drivers are typically so
stupid that they need the brake balance warning even though the pedal travel
has become significantly longer.
PaulH.
----- Original Message -----
But in a dual master cylinder system, it is possible to meet resistance in
one circuit before reaching that point in the other circuit (for example,
in
the case of a failed hose in one circuit). The pressure does NOT equalize
between the completely independent circuits.
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