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

[farna@xxxxxxx]

METERING VALVES:
Don't recall what text I read it from now, but a metering valve requires ~100 psi to open. That's what it takes to overcome the springs in the rear drum brakes. It takes maybe 1/4 of the total stroke to generate 100 psi on the brake system, which generates a total of 300-600 psi depending on component sizes and pedal ratio (adjusted to size of vehicle). If you've rebuilt drum brakes before, it's easy to believe it takes at least 100 psi to budge those springs!

What they do is prevent the rear brakes from locking before the fronts, theoretically. At least all braking surfaces are starting to work at the same time. I'm not sure it really helps by itself. Once the fronts start working weight transfer becomes a factor. With nothing limiting the pressure to the rear brakes they will eventually lock as weight comes off. 

PROPORTIONING:
I agree with Tom. If the rear brakes are sized about right there are no balance issues. In general that means drum cars that are converted to front discs are usually fine. The fronts are improved, but that's mostly from fade. Stock AMC front discs aren't much better than stock drums -- for one or two stops. Discs aren't stronger than cool drums. As discussed on the list just a few days ago, drums get hot fast and braking power falls off rapidly. That doesn't happen to discs. 

I've converted several cars with no rear proportioning device and had great braking. My 63 American had 79 Spirit calipers on front and the stock rear brakes (2" wide, 9" diameter, 13/16" wheel cylinder). The only time I locked the wheels was on a wet surface and ALL FOUR locked. That indicates near perfect balance. I did have the rears lock first a time or two, but that was on ice or wet red clay (which is as slick as ice). I've had that happen with just about every car I've ever had under the same conditions! 

The bottom line is every car will be different! The only way to know if you need a rear proportioning valve or not is to take the car out and test it. You need a wet parking lot with no obstructions or a dirt road (not sand -- you need to get some traction!). Then take the car up to about 45-50 and slam on the brakes as hard as you can. If the rears lock immediately you need to reduce pressure to the rear. If they take a second or two to lock, and they will release when pedal pressure is reduced slightly, you should be fine. While you're at it try turning the front wheels when the rears are locked. Some cars will track pretty straight with the rears locked. Just remember -- the rears aren't doing anything if sliding -- the melting rubber actually LUBRICATES the pavement and makes the rear tires ineffective stoppers.

One note: The proportioning valve doesn't cut brake pressure that much. The 68 TSM states the proportioning valve limits rear brake pressure to 200 psi. The 69 and 70 TSMs state 200 psi for Rambler (American), Javelin, and AMX; 400 psi for Rebel and Ambassador. Remember, total pressure generated is in the 300-600 psi range. Pressure should be around 400 total for the small cars, 500-600 for the large ones, so the proportioning valve takes about 200 psi "off the top".  


--------original message----------------------------------
Date: Sat, 24 Feb 2007 11:12:45 -0800 (PST)
From: Tom Jennings <tomj@xxxxxxx>
Subject: [Amc-list] AMC brake combo valves (waste of time)

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.

--
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)
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