Re: [Amc-list] Grand Unified Brake List

[Tom Jennings <tomj@xxxxxxx>]

> TomJ wrote:
> There are four different combo valves. Many truisms I've heard
> here are not necessarily true:
>
> * Front disc/rear drums, the rears always need/get proportioning. WRONG!
> * Front disc/rear drums, front always gets metering. WRONG!
> -------------

On Mon, 12 Mar 2007, Swygert, Francis G MSgt 436 CES/CECM wrote:

> Enlighten me some more here! The second statement I've found to be true
> -- only a few years used a metering valve.

This really needs (1) complete TSM data and (2) an exhaustive
search, but 1982 has rear proportioning but no front
metering. This appears to be the EXACT OPPOSITE behavior of
metering! The rear prop valve is either hold-off, or more
likely what it says -- proportional; 100PSI in, 90PSI out,
or whatever. In any case pressure to the fronts increases in
time with the rears. With metering, the rears get pressure
first. So the simple truism 'takes up slack in the rear' needs
verification. I doubt everything.

The only way to know is to chop one open unless we find some
harder data.

There's this vague unspoken assumption that front/rear braking
timing is, and must be, exact, but I bet there was a lot of
leeway at low pressures.

This sort of design decision I've never seen written down. The
same is true for computer software. Oddly, it's reasonably
common for electronics, the sort of "here's what I see as
problems, how I am approaching it, and these are the little
oddities around the edges".  I do it for software, but I'm an
idiot. It's actually the most interesting thing about technology
in general and no on ever writes it down, those kitchen-table
conversations that end up as cars or whatever. The Teague
Gremlin on an airline puke bag comes close for visual design.



> I've yet to find a COMBINATION VALVE that doesn't have rear
> proportioning built into it -- at least in an AMC. The TSM may not state
> the valve has it, just that it's a combination valve. The TSMs don't
> spell out things that are considered general knowledge, often just
> detailing a feature the first year or two it's used then glossing over
> it every other year.

I dunno about that -- the TSM spells out quite specifically
the functions contained in the combo valves, which is unusual.
I admit it could be simply not stated, but the TSMs are so
specific here! Here are my notes from the sheet, you can use
them to reference the original text:

VALVE NOTES:

Warn: Manual-reset warning switch. Small, one port feeds both
front brakes.

SR Warn: Auto-reset warning switch only. Small, one port feeds
both front brakes.

SR Combo: 1975 TSM states COMBINATION VALVE -- DISC
BRAKES. Manual-reset warning+metering.  Big, has valve stem,
separate ports for each front brake.

SR Combo3: 1973 TSM states "COMBINATION BRAKELINE PRESSURE
DIFFERENTIAL WARNING LIGHT SWITCH AND PROPORTIONING
DIFFERENTIAL VALVE -- DISC BRAKES".  Auto-reset
warning+metering+proportioning.  Three functions!

SR Combo4: 1982 TSM states "COMBINATION PRESSURE
DIFFERENTIAL/REAR BRAKE PROPORTIONING VALVE", but also that
"two styles are used...differ..external appearance...valves
function in an identical manner". No rear pressure stated.

SR Combo5: 1976 TSM states "COMBINATION VALVE - DISC
BRAKES". Auto (hydraulic) warning switch reset and front brake
metering.  Prop: Rear brake pressure proportioning valve in
rear brake line, located forward of rear axle. Black paint
daub=400psi, blue=200psi.


> But there is still a
> *balancing*, if not a *proportioning*, device used. Stating that no
> proportioning valve is needed/used is a misrepresentation since
> *something* was used to balance braking action.

Umm, I don't think that is correct. Balancing seems to be done
entirely by brake surface area, period, in many years. The
only ones that do it hydraulically are ones with specific rear
proportioning valves, which are a minority.

It wouldn't surprise me to see that the factory installed
different brake shoes in cars that are different from
aftermarket.

I'd have to go back over the brake area tables to be sure, but
somethin like: for an otherwise identical Gremlin vs. Hornet,
front rear brake balance could be finely tweaked by changing
rear wheel cylinder (1/32) or brake shoe area. (The TSMs are
obsessed with this data -- why? It has no bearing on repair
nor maintenance. It's probably internal worry for regulations
and testing that leaks in.) The aftermarket would see little
practical difference in a 5% shoe areal difference, shrug,
and ship one part. Wear, tires, maintenance or lack of, etc,
would swamp the tiny variations that the factory deals with
-- every car goes out with the same shocks, same tires, same
inflation, same brake shoes, same settings, etc. Well, it was
AMC, there's always "top of the pile" :-)



> Almost.
> 65-70 -- four piston Bendix
> 71-74 -- Kelsey Hayes 2.75" piston calipers
> 75-76 (all), 75-78 Matador -- Bendix 3.1" piston calipers
> 77-78 (except Matador) -- big bearing Bendix 2.6" piston calipers (used
> same bearings as big Bendix)
> 79-81 -- small bearing Bendix 2.6" piston calipers
> 82-83 (and through 88 Eagle/Jeep) -- GM 2.6" piston calipers

You're right of course, many of those are the TSMs I don't
have yet!


> The factory 77-78 2.6" calipers reportedly had a phenolic piston, but
> rebuilds use steel.

NAPA breaks out "phenolic" vs... doesn't say (certainly steel)
for AMX/non-AMX but I have no idea why. ALl of the ones I've
rebuilt were steel -- and the set on my 63 Classic came off my
79 Spirit AMX long ago. I bought that AMX used/2 yrs old from
a dealer, and it had steel pistons when I rebuilt them.


> I've never seen four piston Bendix brakes on a six cylinder car from the
> factory, probably seen as a waste, could have been a V-8 only option.

Larger pistons would affect front/rear balance...


> Part numbers, incidentally, don't help a thing when identifying parts
> interchangeability! Numbers seemed to change for no apparent reason for
> parts that I know look identical.

But I wonder if they are identical! We know that sometimes they
are not, and the difference could be something meaningless like
a clearance notch not needed on the older part. But it could
be 1/32" difference in some aspect... but today it's moot,
I'm happy when I can get any part that fits!


> Tom knows I'm not trying to be argumentative -- we had a little
> discussion about these "debates" off-line. Just wanted everyone else to
> know!

Oh yeah, this is Argument in that good sense -- we want to
make good data, that's the goal. No one likes to be wrong,
but I'm glad to be wrong when it fixes stuff. The whole is
greater than the sum of it's parts and all that crap.
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