[Amc-list] Stop! With the brakes already!

[Tom Jennings <tomj@xxxxxxx>]

This is twice -- we're both wrong again! We understimate the
number and kind of parts AMC juggles under the cars! I think
a lot of test-track tweaking went on, as the changes are very
fine-grained and they bothered to write it all down.

On Fri, 23 Feb 2007, Swygert, Francis G MSgt 436 CES/CECM wrote:

>> So far, none of them contain a proportioning valve -- when
>> those are used they are separate and on the rear of the chassis.
>
> --------------
> This is incorrect! ALL combo valves have a proportioning valve built
> into them for the rear brakes. Combo valves were only used with disc
> brakes. You need to read a little more on this -- see the referenced TSM
> above.


There are AT LEAST TWO TOTALLY DIFFERENT disc brake "combination
valves".

1973 and 1974: specifically states that it contains three functions:
1. press. diff. warning light switch
2. front brake metering
3. front/rear brake proportioning valve.

The various functions are discussed in detail. It's not a typo
or mis-read. Unlike other years I have access to, there is no
cutaway drawing of it.


1975: TSM specifically states "a two-function combination valve...":
1. press. diff. warning light switch
2. front brake metering

Again it's functioning is discussed in detail. 75 (and other
years) do have a cutaway drawing. If it's accurate, there is
no rear-pressure proportioning valve.

I'm not saying only 73, 74, 75 have these weird issues, just
that I'm not done making tables yet.


There are also two types of "pressure diff. warning light
switches" (eg.  for drum brakes) too, and maybe more. Different
years have different specs for the pressure difference at which
the light goes on -- though that could be nothing more than
what that batch tested at. (I also don't think it matters at
all; brake hydraulic problems are usually leak/fluid loss or
mushy with air, and it's easy to get 80 - 100 psi difference,
and I doubt really 70 is OK if 90 is bad...)



You'd think that 72, 73, 74, 75 would be the same in a given
chassis/engine/trans combination -- there are many small
differences.

It's really surprising how many differences there are. It seems
to heat up in 1970. The 70 TSM doesn't even spec brakes hardly
at all! The 60's cars, esp. early, seem quite straightforward.

I plan on getting a current part number for every wheel cylinder,
shoe, caliper and rotor and see how THAT matches up. I bet
that the aftermarket replacement industry has flattened a lot
of the changes into one part for multiple years, etc.



One hint of subtlety is that pretty much every TSM states front
and rear braking (shoe, pad) area. That plus (PSI * slave area
is what's being fiddled by the factory.
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