[DeTomaso] Brake Upgrade

[MikeLDrew at aol.com]

In a message dated 11/11/11 13 15 58, JDeRyke at aol.com writes:


> 1)- totally remove the stock non-adjustable proportioning valve that 
> limits
> the front brake pressure, so you rely mostly on the toy rear calipers.
> Removing this and re-plumbing for full use at both ends allows the decent 
> stock
> front brakes to work to full capacity. This alone improves brake power
> markedly and is all some guys decide they need.
> 

This is a complete misconception fueled by a fundamental misunderstanding 
of how a braking SYSTEM works.   This modification will improve the 
performance of the front brakes and screw up the performance of the braking SYSTEM as 
a whole.

The original calipers were badly mismatched, front-to-rear.   I have copies 
of the original braking system DOT certification tests, conducted on 
hand-built preproduction prototype Panteras (chassis number 1007, 1010, etc.), 
where the need for the proportioning valve was revealed, and then the braking 
improved once they were fitted.  

Brakes need to be properly balanced front-to-rear.   Especially in a 
mid-engined car with extensive weight on the back end, and even more especially in 
one fitted with aftermarket wheels and tires (where typically the 
difference in front-to-rear tire width is even greater than stock, and thus the need 
for rear braking is greater than stock), it's crucial that the back brakes 
contribute their fair share to the overall stopping scheme.

The stock proportioning valve is actually a pressure reduction valve.   It 
is designed to reduce front line pressure, relative to rear line pressure, 
so that the front and rear brakes are properly balanced.   

If you remove this valve entirely, as Jack recommends (again and again), 
what you are left with is overly-sensitive front brakes.   To the ignorant, 
this is an 'improvement', because for a given amount of leg pressure applied 
to the pedal, there is a greater apparent braking effect.   As long as you 
are just driving around normally and not attempting to actually stop the car 
with any sort of vigor, this can feel like 'good' brakes.   It's quite true 
that a car so-modified will stop with less effort when coming to a stop sign, 
etc.

However, what this modification has done is actually to disable the rear 
brakes!   That is, the front brakes are now so sensitive, relative to the 
rears, that when pedal pressure is increased, the front brakes simply lock up 
and the tires slide, while the rear brakes are hardly doing anything.

Real-world performance braking (track work etc.) will reveal that cars with 
such a modification have substantially LESS overall stopping power, because 
they are relying almost entirely on their front brakes.   This also will 
lead to rapid overheating of the front brakes and resultant fall-off in 
braking performance (which is what Brent experienced I believe, as I think a 
previous owner of his car followed Jack's advice).

If the stock pressure reduction valve is removed for any reason (on a stock 
braking system), then it should be replaced with an adjustable 
proportioning valve.   Initally the valve should be plumbed wide-open (no restriction), 
then real-world testing should be conducted with progressive adjustment, and 
repeated hard stops.   A hand-held heat gun (thermometer) can be useful, as 
rotor temps will give an indication of braking performance--if your front 
rotors are hot and your rears are not, you don't have enough rear brake bias. 
  

Eventually you'll go too far and the rear brakes will lock up first; then 
back off the valve so that the fronts lock up first BUT the rears are still 
doing their fair share of the work.   Too much rear brake bias is dangerous; 
if the rear brakes lock up, the car can easily go out of control.   So the 
front brakes should always lock up first--but not excessively so.

Jack's advice regarding brake vacuum is quite valid.   If your power 
booster is blown out, or if you have insufficient vacuum to operate the power 
brakes, the brakes won't work well at all.   Some cars can have great braking 
effect with no power assist (the Ferrari F40 is such an example), but this is 
achieved through proper engineering--typically the pedal is much longer, so 
the leg has a much greater mechanical advantage over the master cylinder.

A properly functioning Pantera brake system, equipped with performance pads 
but no other upgrades, should be able to lock up all four wheels.   Sadly, 
many Panteras are suffering from some form of deterioration and this can't 
be achieved.   Rather than trying to band-aid the system, the responsible 
thing to do is identify the actual source of the problem so that the stock 
(which is to say, quite good) brake performance can be restored.

All bets are off when aftermarket systems are installed.   Ideally, the 
vendor will have done his homework, and front-to-rear proportioning will be 
handled via different piston sizes in the calipers (my Wilwood system from 
Dennis Quella has four calipers that externally are identical, but differ in the 
sizes of the pistons on the front versus rear calipers for just that 
reason).   If you elect to try to piece your own system together with various bits 
and bobs, you can still achieve success.   You'll probably have to resort 
to some sort of adjustable valve to do so unless you're smart enough to 
succesfully calculate the necessary piston sizes front and rear to achieve the 
desired proportioning (which I most emphatically am not!).

Mike