[Amc-list] AMC brake combo valves (waste of time)

[Tom Jennings <tomj@xxxxxxx>]

Here's what I am working out:


The valving (in all cases, a cast block bolted to the lower
left firewall) varies all over the place. And strictly speaking,
it doesn't seem absolutely necessary. I know that sounds wrong,
but consider this:

There are only three total functions provided by the valving:

1. front/rear pressure difference warning (no effect on braking)
2. front-disc-pressure metering (hold-off)
3. front/rear proportioning.

#1 has no effect on braking; we can ignore it here. All valves
provide it.



#2, metering: The TSMs of years that provide this (that I
have TSMs for) state that it restricts brake fluid flow to the
front calipers such that the slack in the rear shoes is taken
up before full pressure is applied.

Yet many (AMC) disc-brake cars don't have this, even within
the same chassis/engine/brake combinations!

It's not calibratable; it's just a spring inside, and there's
no test for it, no spec on pressure, no easy or specified way
to tell if it is working or not.

Metering would only have an effect at very low braking forces,
and probably helps balance front:rear pad/shoe wear. If you
mash the pedal hard it has no effect. Without it, you probably
have slightly increased front pad wear (eg. light pedal pressure
engages the front calipers before the rear shoes rub).


*** Can any one generate HARD DATA here on front metering,
     on the car? Can you think of a way to check, or test the
     front-holdoff? I can't. On a bench a "simple" pressure source
     could see at what pressure the front meter valve starts to
     open, and that plus piston size blah blah you could guess
     rear shoe movement. But how much do valves vary from unit to
     unit? Do they weaken/change with time?  Who knows? Who cares?

With properly adjusted rear brakes it's probably very, very
minimal, and a "cost saving passed on to us" when AMC saved a
few pennies and dropped itin later years.

Personally, none of my retrofitted cars have ever had a metering
valve; my 70 Hornet, with late Bendix fronts and 9x2 or is it
9x2.5 rears and the right wheel cyls, is *perfectly* balanced
wet or dry. (I installed a self-resetting warning switch; I wish
I hadn't bothered, and had just plumbed the brakes 60's style --
from the master to the brakes.)

My 63 Classic likewise -- I have Bendix discs and the right size
rear cyls (admittedly by accident) and balance is near-perfect.
No valve of any kind.




#3, proportioning: For our street AMC/Ramblers, with donor'ed
disc fronts and drum rears, if you pick the right wheel cylinder
you don't need proportioning. Period.

If you're autocrossing or racing OK go get an aftermarket valve
and adjust to make yourself happy. Otherwise it's a waste of
time on a retrofitted car for which you can buy wheel cylinders.

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