<VV> > Shuttle Valve Possible correction?

[BBRT]

Reviewing various sites leads me to believe the GM valve is NOT a shuttle 
valve but may be merely a switch. None of the descriptions describe a 
function of that particular block to block fluid flow. I am thinking that 
maybe the only place the safety feature is done is in the MC itself. So, not 
sure.

If it is only a switch for the indicator light, then the MC is at fault in 
the original post.

Chuck S

From: "BBRT" <chsadek at comcast.net>


> 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