[DeTomaso] Poor brakes

[MikeLDrew at aol.com]

In a message dated 7/10/09 11 05 11, JDeRyke at aol.com writes:


> To clarify: if a pad wears, more fluid is used in that part of a caliper
> during braking which moves the shuttle, indicating pad wear, as well as 
> what
> you describe. 
> 
>>>Again, you aren't getting how this thing works--not even close.   The 
shuttle valve is not displaced because more fluid has flowed into the chambers 
of the calipers whose pads have worn.   It is a differential pressure 
valve, plain and simple.   When you step on the brakes, you generate some rather 
high pressure (1000 psi?   I don't actually know).   If you suffer a 
rupture, you will have that high pressure on one side of the valve and zero psi on 
the other, and that differential pressure is what displaces the valve.   The 
valve actuates a switch which illuminates the light on the dash.

The brake wear indicators are metal tangs on the backing plate of the 
original brake pads, which contact the rotor and illuminate the same light on the 
dash.

Although they happen to use the same light, the two warnings are totally 
unrelated to one another.


> I know you like this infernal device,
> 
>>>Actually, I very much *don't* like it.   I advocate its removal, and in 
fact my car hasn't had it since forever.   But I advocate its removal in a 
proper manner, keeping the front and rear brake systems totally isolated from 
one another.


>  but it really needs the
> modification that AMC and others did- adding a spring on both ends of the
> bore to help return the shuttle from the ends of the bore. 
> 
>>>Yes, that would be very helpful if you chose to keep it--but I think 
it's far more trouble than it's worth.


> The trouble often
> starts when one bleeds the brakes and the motion of the fluid bled out 
> drives
> the shuttle far to one side where varnish sticks it in its bore.
> 
>>>Again, you are confusing fluid flow with pressure differential.   When 
you open a brake bleeder nipple, you all but eliminate the ability for that 
caliper to sustain any brake pressure at all; the pressure that is generated 
is consumed by pumping fluid overboard--which is exactly what happens in the 
event of a line failure too.   During brake bleeding (assuming you are 
bleeding one caliper at a time) the pressure differential is causing the fluid 
to flow on only one side of the valve, but it isn't the actual flow that is 
creating the movement of the valve--it's the difference in line pressure.   
Gradual fluid flow (as when the brakes are applied, or when the pads wear 
over a long period of time) when the pressure remains constant will have no 
effect on the valve.


> > I've
> cleared several shuttles that were stuck solid, Larry Stock has done the 
> same in
> his shop, and Roger Sharp has found at last one during Nor-Cal Tech 
> sessions.
> I believe its an endemic problem to the Pantera best fixed by eliminating
> the thing entirely. My 2¢- J Deryke
> 
>>>And on that we are in complete agreement! :>)

Mike



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