Re: [Amc-list] rear wheel lock up problem still

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

What you are describing is technically called a "combination valve". You
have a three-way type -- metering, proportioning, and brake warning
light switch. Two-ways leave metering off. Easy to tell apart -- the
three-way has two inlets and three outlets (one for each front brake),
the two-way two ins and two outs (only one for one of the front brakes).
With a two-way a T in the line serves the other front brake. The T is
usually before the combo valve but can be after. Combo and single
function proportioning valves are typically only used with front
disc/rear drum setups, but you can use a two-way or proportioning only
valve with drums. When both ends act the same a balancing device isn't
needed, unless things have been changed around. Then you might want/need
to adjust balance. 

The metering function won't allow the front brakes to engage until ~100
psi is in the line. That's what it takes to overcome the springs in the
rear drum brakes. In your case, Dave, that's likely why you're getting a
little quicker lock-up. The proportioning function of the valve you have
obviously allows more pressure to the back than you need. If they only
lock up when you "stand on the pedal", you might be okay. You need to
find a really wet but open parking lot and make a few normal then
progressively harder stops to get the feel for it. If it only locks on
really hard, near panic stops you should be safe. Try turning a little
when you lock the rears and see what happens. The Rambler might be light
enough in the back that it want to swing around on you in that
situation. You NEED TO KNOW!! Then at least you'll know to let off the
brakes a little as soon as you feel the lock. Let off pressure to stop
faster is contrary to natural reaction -- can you do it? A lot of people
can't. I'm not panic prone and generally can, but there's that first
second or two no one can help, and that will be to much if the car
switches ends easily. Most Ramblers don't -- my 63 American would track
pretty straight even with the rear wheels locked and the fronts turned
slightly. It took a good bit of turn to make the back end feel like it
might come around. Surprising with a short wheelbase car! 

The only way to fix your situation is to use an adjustable rear
proportioning valve. Go to a wet parking lot and adjust until the rear
brakes will only lock in the hardest stops. I don't adjust until they
don't lock at all, just until I have to put all I can in it to lock
them. That leaves plenty pressure before locking to stop. Once you get
there on wet pavement, try it on dry. The rears shouldn't lock at all
(unless fronts lock too) on dry pavement, maybe if you put both feet on
the pedal and try to pull the steering wheel off at the same time, but
that should lock all fours. (Been there, done that, turned the seals in
the master cylinder -- an old single "fruit jar" on a 61 American --
inside out!! Almost stopped in record time too, the seals flipped about
a foot from the bumper of the car in front of me. I thought I'd been hit
in the back and pushed forward -- sure felt like it!!)

There are other problems you could have. THE MOST IMPORTANT THING TO
REMEMBER WHEN MESSING WITH BRAKES IS YOU'RE MESSING WITH YOUR LIFE --
AND OTHERS ON THE ROAD!! Test brakes on a clear wet surface where you
won't hit anything. A dirt road will do in a pinch -- not as good as wet
pavement but a very close substitute. Icy surfaces WON'T do though --
TOO slick!!

1) A rear brake line could be pinched or partially blocked, causing a
rise in pressure. A blockage in one line (to one side of the T on the
axle) could still cause both to lock. Check the rubber line too --
replace it if original by all means!! 
2) Have the rear wheel cylinders ever leaked, or do they leak now? If
the brake shoes are contaminated with brake fluid they will grab. You
can try brake parts cleaner, but most likely the shoes will need to be
replaced if they got much on them. You could have a minor leak that's
just enough to coat the shoes and cause sticking, but not enough to leak
out on the ground. Pull a drum and see. If there's a greasy mass around
and just under the wheel cylinder, and the brake shoes look dark, that's
the problem. Just don't have someone press the pedal with the drums
off!! Sounds like a good idea, and it can be done IF that someone
pressing the pedal knows what's going to happen if they press to far and
don't stop as soon as you say, but there won't be enough pressure
build-up to see a leak. The pressure goes to the easiest to move side,
which is the side where the shoes have the least resistance (without a
drum).
3) The master cylinder could be too small. All AMC drum brakes that I
know of (58+) use a 1" (or metric 0.9xx") master cylinder. So do the
2.6" 77+ calipers. The bigger 3.1" Bendix and 2.75" Kelsey Hayes
calipers use a 1-1/8" master; though non-power use a 1-1/16" --
specifically to provide a little more pressure/less foot effort. So the
brakes can be "adjusted" by changing master cylinders. A smaller bore
will increase pressure; a larger bore will decrease pressure. Stroke is
about the same, but a larger bore does move a little more volume per
stroke. You MUST remember that when you change the master cylinder you
change FRONT AND REAR braking power. This can cost a bit, because you
really need to try another master cylinder, preferably in 1/16"
increments, then hit that wet parking lot for testing BEFORE normal
driving. A 73 Matador w/non-power disc uses a 1-1/16" (1.0625") master,
power disc uses 1-1/8" (1.1250") -- refer to
http://www.tocmp.com/manuals/AMC/1973/Service/Chapter8html/browser.htm. 

In Dave's case, I wouldn't think a larger master cylinder would help.
The front brakes don't lock, partially because discs are harder to lock,
but sounds like the fronts are fine as is. If the rears lock only under
the hardest braking, I'd leave them alone. If they lock during normal
but hard braking, and the lines are okay, you'll be safer with an
adjustable proportioning valve. If the car doesn't have a tendency to
switch ends with the rears locked and front wheels turned a bit, you
should be okay though. You can force the rear around, but it shouldn't
do it easily with the rears locked ON WET PAVEMENT. Testing on dry
pavement won't prove much -- that's usually not a problem at all. It's
when things are a little slippery (wet pavement) that you're more likely
to have problems with a panic stop. Just remember to test, make
adjustments, and then test again BEFORE normal driving. 

It's easy enough to take it easy driving to a test area, but you NEED TO
KNOW how the car's going to react in a panic stop before regular
driving. This is why most people are real careful about making changes
to the braking system. You have to understand what's going on, enough to
make logical decisions before making changes. You can easily kill
someone if you don't. It's just about the same with any modification you
make to a car, but brakes and steering are the touchiest. If the engine
dies even in the middle of a high speed run you're not likely to get
hurt. Brake or steering failures are more likely to be fatal. So be
careful and think things out before making major alterations, like
changing master cylinders and brake types. 

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