<VV> Shuttle Valve? - Probably not the master cylinder

[BBRT]

It is my understanding that the block is a shuttle valve which has a rod 
with o-rings on each side. In the rod center is a groove which a plunger 
attaches to which will cause the indicator to light if it goes one way or 
the other. Upon one circuit failing, the rod goes to one side and blocks the 
fluid from going to the other  brake circuit, if working correctly.

So failure modes are;

rod is stuck and won't go to one side.
o-rings (they may be square seals) are shot.

Or, one could have been driving on fronts or rears only and it has been bad 
for some time. This should show up in bleeding brakes.

There is another failure mode of the MC itself where the seals are bad 
between the two lower volumes and the MC just pushes past and doesn't apply 
pressure to the other side. Had such a failure on a '69 Toro many moons ago. 
Rebuilt MC and it solved problem.

BTW, very good reasons to have an operable functioning emergency brake.. You 
CAN drive home on it.....

Chuck S
BBRT