Re: [Amc-list] 1970 brake specs?

["Swygert, Francis G MSgt 436 CES/CECM" <Francis.Swygert3@xxxxxxxxxxxx>]

Date: Thursday, February 22, 2007 08:19 PM
From: Tom Jennings <tomj@xxxxxxx>

>> Here's one for you: 1975-up brake "combo valve":
>> * auto-resets the warning switch after bleed or repair


>> Does that last mean you don't need the check valve in the master
>> cylinder port for the rear brakes?

> No! ... which will remind me to Read The F'ing Manual before opening
> mouth...

>>On Thu, 22 Feb 2007, Swygert, Francis G MSgt 436 CES/CECM wrote:

>> NEGATIVE!! All it means is that there's a little pull-out spring
loaded
>> stem on the left end. The spring is NOT inside the brake line port.
It's
>> part of the proportioning valve. After the preset pressure is reached
>> the valve won't let additional pressure pass. The spring effectively
>> cuts off flow to the rear brakes after the pressure setting is
reached.

>We're both wrong...

>It's a "front [disc] brake pressure hold off valve". It is
>internally sprung; it delays hydraulic pressure to the FRONT
>brakes until the pressure is high enough to take up the slop
>in the rear shoes.

>The TSM states the stem must be pulled out (spec in book)
>to bleed the brakes. Not needed for vacuum bleeding, at least.

-------------
Hey, you mis-led me with your statement about proportioning valves!! ;>
Well, I can't blame you though (I was partially kidding above). I
vaguely recalled something about the METERING (front brake hold-off)
valve having a stem, but only vaguely, so I went with your suggestion
about the proportioning valve... I didn't look at a book either, and
this is in the 73 TSM on-line at
http://www.tocmp.com/manuals/AMC/1973/Service/. 
-------------

>So far I've found at least two substantially different DISC
>BRAKE COMBINATION VALVE types and at least two BRAKE WARNING
>VALVE AND SWITCH types.

>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. Note that a combo valve (or proportioning valve) isn't required
with four wheel discs (or four wheel drums) either. Seems that the
different characteristics of disc and drum brakes is what requires
balancing, which makes sense. 

There were two combo valves -- one with and one without metering. The
metering function was eliminated from the small cars around 77 -- I
THINK (not 100%, especially not without TSMs in hand!) it was with the
advent of the smaller piston calipers and corresponding smaller bore
master cylinder. The big cars continued to use the three-way combo valve
through the last one (78). Easy way to tell: the line from the front
brake portion of the master cylinder has a T in it for the left front
brake before the combo valve if it's a two-way (prop and switch -- only
four brake line connections), three way (prop/switch/metering) has both
front brakes going into the combo valve (five brake line connections).
Also take a look at the combo valves and tech info at www.mpbrakes.com.
They have details on the repro GM combo valves that are what AMC used --
at least appearance wise for the late 70s big cars.
--------------

>The two major types are brake warning switch manual-resetting
>and self-resetting. 75-up are self-resetting (will know for
>sure when I get all the TSMs, but it seems a safe bet).

--------
I've noticed this also. 
-------

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