[DeTomaso] Poor brakes

Rick rwgushue at verizon.net
Sun Jul 12 02:23:40 EDT 2009


   I agree with most of this.. but the proper name is a Pressure 
Differential Valve not a flow whatever valve.  It operates when pressure 
is lost on either side of the system. Yes there is a very small amount 
of flow when the caliper pistons are moved outward but not enough to 
move the valve inside of the PDV as the pressure is the same on both 
sides so no movement. The part inside is a spool valve and in operation 
it "shuttles" towards the low or no pressure side. It has a "point" on 
both ends that shuts off the low/no pressure side, this is when the FLOW 
really occurs just a bit to shuttle the inner spool valve closing off 
the low side. It is a spool because the center has a groove that will 
operate the pin that causes the brake warning light to come on when the 
spool moves to either side. Some of these have a small spring on either 
side to help keep it centered. When the pressure is lost on one side the 
resulting pressure overcomes the small spring and closes off the low/no 
pressure side. As posted earlier this valve lets you keep either the 
front or rear brakes operating at least good enough to stop safety.
   Now if you cut all four flexible brakes lines at the same time I 
don't think the valve would operate at all but this would be the least 
of your troubles. Cars now days have a combination valve but that's 
another story.

Rick
1582

asajay at asajay.com wrote:
> I agree with Jack, it really -is- based on flow.
> 
> The system should be static for the most part.  When at rest, the  
> master is retracted and the calipers push fluid back to the Master  
> cylinder.  When the pedal is applied, pressure is increased -AND-  
> fluid is -moved- into the calipers.  Yea, it's not a lot but it -is-  
> moving fluid, therefore, flow.
> 
> When one side experiences a rupture, a pressure differential is  
> created because one side is flowing rapidly against no pressure while  
> the other side is flowing slowly against a higher pressure.
> 
> If there was -no- flow, the shuttle would not move.
> 
> Now, if it was a totally enclosed system.  If you increased the  
> pressure on once side it should move the shuttle versus the lower  
> pressure on the other side.  However,  I've been told that liquids do  
> not compress, so no matter how high you change the pressure on one  
> side, in a totally enclosed system, I don't think you'd get any  
> movement of the shuttle.
> 
> But our brake systems by nature of the master cylinder and pistons in  
> the calipers are not static.
> 
> That's my story, and I'm stickin' to it.
> Asa Jay
> 
> Quoting JDeRyke at aol.com:
> 
> 
>>In a message dated 7/10/09 11:16:19 AM, forestg at att.net writes:
>>
>>
>>>The amount of flow has nothing to do with moving the shuttle....
>>>
>>
>>I disagree, but its not worth fiddling with to verify- at least not on our
>>car where the valve is dust-covered in a box in the garage. Cheers- J
>>Derryke
>>
>





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