Proportioning Valve Placement

I wrote an extensive article on AMC disc brake balancing systems in AMC Magazine, Issue #19, Summer 2011. Here is a summary of what AMC used for balancing and when:

YEAR(S)     SERIES        DISC BRAKE BALANCING DEVICE

1965-67    10, 80         Non-servo rear brakes

1967           01             Non-servo rear brakes

1968          10, 80        Non-servo rear brakes

1968-70     ALL            Proportioning valve only

1971-74     ALL            Three-way combination valve (pressure/metering/proportioning)

1975-76     ALL            Two-way combination valve (pressure/metering)

Early 77      10             Two-way combination valve (pressure/metering)

1977-78     ALL            NONE - Pressure differential only

1978         40 (4 cyl)    Two-way combination valve (pressure/proportioning)

1979        01, 60          NONE - Pressure differential only

1979       40 (6 cyl)      NONE - Pressure differential only

1979       40 (4&8 cyl)  Two-way combination valve (pressure/proportioning)

1979       AMX (6&8 cyl) Two-way combination valve (pressure/proportioning)

1980       60                  NONE - Pressure differential only

1980-87    ALL              Two-way combination valve  (pressure/proportioning)

Want more information on brake balancing theory?

Note that "pressure/..." actually means "pressure differential/...". Not enough room in the original chart to put it all in.

If you'd like to see the entire article send me a PM with your e-mail address and I'll send it. It's three pages of text and one of illustrations, a bit much to post here!

tomj wrote: oh and another thing most people miss when they put really tall rims on cars is that the brake rotor size needs to increase as well. the ratio of rim to rotor radius is a measure of how much leverage the wheel has over the brake -- large wheels have a longer lever and put extra load on the brake.

Great point! But, don't forget to add that *generally* a larger wheel also increases the rotational inertia and CG height as well. Brakes stop the mass of the car and the rotational inertia of all connected rotating components. Greater wheel inertia puts even more load on the braking system.

CG height impacts front/rear weight distribution under braking/acceleration. Too much change (under braking) and you may be able to lock the rear tires when before you could not due to a lighter rear end under hard braking. (ie. proportioning valve is no longer valved correctly for the system)

Just some more interesting thoughts! ...or down the rabbit hole we go?...

lol. he might be a good administrator. he's good at cars too.

"Though my 18" rims weigh 2 lbs more than the 17" rims I had. The big difference is tire.  Though, the tread diameter is the same as stock. What makes it really noticeable is getting up into the 19" and 20" rims with 15" and 16" discs. Thats  if you can fit them proeprly in the arch or suspsension. Modernising does not mean better, just looks cooler."

I'm a bit surprised the larger wheels weigh more, but depends on what they are made of and the style. More goes into looks rather than weight savings. The original purpose for "mag" wheels was to save weight, but those originals were made of magnesium (hence "mag" wheels) at first and expensive, now they are all aluminum and usually weigh as much or more than steel wheels. The reason I'm surprised your bigger wheels weigh more is that hte rubber diameter is the same -- more wheel, less tire sidewall height. The rubber tire is usually a bigger weight contributor than the wheel.

"Looks cooler" is subjective! I don't mind the 16" wheels at all, 17 & 18 are getting a bit big on the old cars to me, anything over 18" just doesn't look right at all. You're showing your age in saying the big ones look cooler... but then maybe I'm showing mine saying they don't!!