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9” wheel cylinders in 11” rear drums?

53Flattie

53Flattie

Member
Joined
Sep 23, 2002
As the title implies - it seems like I’ve read that it’s more beneficial to run the 9” wheel cylinders in the rear of an 11” conversion because of the small diameter of the wheel cylinder.
(This is on a single-pot MC on my 3A. Not my CJ5)

Am I remembering this correctly?
 
Run the cylinders that are designed for the backing plate. Use an adjustable proportioning valve if needed.
 
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The more I think about it - maybe I’m remembering the possibility of mis-matching 9” WC’s between front and rear. Or something like that.

I have the correct 11” WC’s so I’ll use them.

Just over here second-guessing. Nothing to see here!!!
 
53Flattie said:
he more I think about it - maybe I’m remembering the possibility of mis-matching 9” WC’s between front and rear.
Click to expand...

I'd say so, the wc's are not physically interchangeable. You can get different diameter ones for front & back thet fit the 11" plates that will help a bit with the front/rear balance; thse were used on the 70's era AM postal jeeps but you will probably need a brake balance valve inserted in the rear line.
 
i'm running 11" drums all around from an intermediate cj5. fronts have 1.125" bore wheel cylinders, rears have 0.9375" bore.
 
'75 and newer CJs have a proportioning valve like Howard describes.
Its purpose is to hold off the rear axle braking pressure in hard stops.
As you brake harder, weight transfers more to the front axle and the force needed to lock up the rear decreases ... proportionally :coffee:
 
I run giant k10 calipers with a stepped bore master cylinder and a proportioning valve. The goal was to have even holding power offroad but cut the overall clamping force on the rear calipers when applied heavily.

There are several variables that one can manipulate for different results but you need to think of it like a distributor curve. The flyweights and springs adjust the curve differently and you want a curve that follows the engine. You don't just put a lot of weights in and call it good.

Brakes are just two distributors one on the front circuit one rear your pedal force is your rpm and your wheel pressure is the overall advance. Your proportioning valve is the vacuum advance. Master cylinder bore is opposite the flyweights like the springs and the wheel cylinder bore is similar to the flyweights.
The key difference is you have two different curves with the same master cylinder.
 
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