[DeTomaso] Poor brakes

[MikeLDrew at aol.com]

In a message dated 7/9/09 20 55 2, JDeRyke at aol.com writes:


> The shuttle
> can be safely thrown away and the various lines hooked together. Its only
> job is to warn you of worn brake pads, and only if you still have pads 
> with
> ground wires embedded.
> 
>>>You misunderstand the function of the shuttle valve completely.   It has 
absolutely nothing to do with the brake pad wear indicators.

There is a red idiot light on the dash that has two unrelated inputs (later 
cars had a red and an amber light with separate inputs).  The stock brake 
pads had tangs which would contact the surface of the brake rotor when the 
friction material wore down to a certain point (which would take a million 
miles since the stock pads were so hard).   When that tang touched the rotor, 
it completed the circuit and illuminated the light.

In the event of a misfortune where you ruptured a brake line or a brake 
caliper seal let go, differential pressure would cause the shuttle valve to 
displace, and in the case of the early cars, illuminate the same light.   
Chances are the fact that you were sitting upside down in a tree would be 
indicative enough of your braking problem, and you wouldn't need the additional 
reminder offered by the light however. :>)

While it is true that the shuttle valve system can be safely removed, it's 
important to remember to keep the front and rear brake circuits separated.   
The two circuits operate at different pressures to accomplish optimum 
stopping, and additionally, the fact that they are independent offers a measure 
of safety in the event of a catastrophic failure on either the front or the 
rear system.   Although the pedal will likely mush towards the floor, there 
will still be *some* stopping going on.   Merging all four lines into a 
single common fitting (which is what happens if the shuttle valve is dismantled, 
removed, and then the gutted housing is put back into place) defeats all 
that.


> >Finally, if you also throw away the stock non-adjustable proportioning
> valve, ALL the brakes will run at full power rather than having the fronts
> hamstrung as they are stock. 
> 
>>>Which could potentially be a good thing, but probably not.   What that 
means is that the front brakes will now be overpowered, and the sensitivity 
will increase.   This provides the illusion of greater stopping power, and 
it's true that for a given amount of leg pressure, under moderate braking 
circumstances, perceived braking performance will improve.   However, by 
increasing the effectiveness of the front brakes, you are essentially disabling the 
back brakes, so the overall effectiveness of the total brake *system* may 
be seriously compromised.   When serious braking is called for, the front 
brakes will lock prematurely while the back brakes do next to nothing.

When you are sliding under the back of a semi with smoke rolling off the 
locked front tires and the rear tires merrily coasting along, the fact that it 
required less leg effort to achieve this state of affairs   will come as 
small consolation. :<(

Many Mangustas are afflicted with exactly this type of problem.   During 
the course of production, a proportioning valve was introduced into the rear 
brake circuit, making those cars exceptionally prone to premature locking of 
the front rotors.   Steve Liebenow will be able to discuss this further; I 
believe the prop valve appeared initially, then disappeared, then came back 
again?   Or something like that.   In any case, it wasn't well thought-out at 
all, as period road tests and individual anecdotes will attest.

(In fact the former owner of Steve's Mangusta wears a scar on his chin from 
the steering wheel, gained when a Toyota truck turned left in front of him 
and he slid into the front of it).


> >Deleting the prop-valve and the shuttle valve are
> often the only brake upgrade most street Panteras need. And its pretty low
> cost.
> 
>>>Assuming the other brake components are kept stock, probably the best 
way to go about monkeying around with the brake system is to plumb in a 
manually adjustable proportioning valve into the front circuit, replacing the 
fixed-orifice stock valve.   This allows you to compensate for the vast 
differences your Pantera likely has between its original and current front/rear tire 
size ratio.   In fact, with mondo rear tires now the norm, the rear brakes 
are capable of contributing even greater overall stopping effort than when 
the cars were new, which is an argument for increasing the reduction of brake 
pressure to the front brakes, not decreasing the reduction.

At the same time, if the shuttle valve system is proving problematic (which 
it often does, particularly when brake systems are neglected as they are 
all too often), it can easily be bypassed by connecting the rear 'in' and 
'out' lines together, and the front 'in' and 'out' lines together.   A pair of 
T-fittings does the job nicely; a brake switch can be incorporated into the 
rear brake system T, and the front brake input is split to the left and right 
calipers.

Cheers!

Mike


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