Brake proportioning systems/schemes/designs/setups

from the factory parts manual:
1974-1975 did have a prop valve from the factory
Jeep p/n 5352422

as near as I can tell, 1973 and prior CJs did not have a prop valve.
they did have a 3 or 4 way tee which included the brake safety switch for failure lamp at the dash; from what I can tell, this was not a prop valve.

on early CJs, there appears to be a prop valve for the rear brakes only, located towards the rear of the Jeep; this includes Jeepster as well.

from my experience, Jeeps with drums brakes used different diameter wheel cylinders, between front and rear brakes. Typically, the diameter of the front cylinders would be 1" or 1-1/8" diameter, while the rears would be 1", 15/16", 7/8", 3/4" etc depending on the vehicle application or size of the brakes.

Although there are different part numbers for the cylinders, one difference might only be the location of the bleeder screw, angled or straight.

I'm sure some folks innocently mistake these 3 or 4 way tees for some type of proportioning valve.

 

Does anyone have a suggestion as to a wise place to mount a proportioning valve on the frame? I got the kit from Herm and asked him and he said “anywhere you want.”

 

To follow up on one of Tim's suppositions, Jim pretty much answered the question of proportioning braking via larger front wheel cylinders as compared to the rear. As an example, the F134 Tuxes got 10" drum brakes with 1 1/8" front cylinders with 1" rear cylinders. This model Tux also had a "self energizing" setup that caused the shoes to cam into the drum as the brakes were applied. This made for suprisingly powerful brakes when they were adjusted properly.
-Donny

 

I'm glad this this is brought up seperate.
I had just done the 4 wheel disk swap when the towbar came loose. my muscle memory of the 9" brakes with disks and no proportioning valve brought the rear around darn quickly.

My question would be the advantages of the adjustable wilwood valve vs the typical stock valved setup that references the pressure on the fronts.

Does the adjustable setup only limit total pressure to the rears or does it reduce the pressure even at partial pressure at the pedal?

 

It's a little bit more complicated than that. The ideal pressure ratio from front to rear varies with how hard you are pressing on the brakes. Press on them harder and you get more weight transfer to the front and the rears need a lower relative pressure.

Just having a cylinder size difference front to rear always has the same front to rear pressure ratio and doesn't fit the ideal pressure ratio well. To prevent rear lock up, you need to have the rear pressure low enough it doesn't lock up under the hardest braking. This leaves the rear pressure lower than it should be for normal usage.

To combat this proportioning valves were created and have a "knee-point" in the front to rear pressure ratios. This allows a better fit to the ideal curve.

Adjustable proportioning valves have the ability to move the "knee-point" up and down. I'm sure my words don't explain it well, but thes graphs in the 4th post here: Learn me: adjustable proportioning valves.| Grassroots Motorsports forum | show what I'm talking about.

The pink line in the first graph is the ideal curve. It also shows a non adjustable proportioning valve curve with the knee-point that pretty well approximates the ideal curve.

The second graph is from an adjustable proportioning valve manufacturer showing how the knee-point moves up and down as you adjust the valve.

All that said, you get better braking with a proportioning valve instead of only having different sized wheel cylinders front to back, and an adjustable valve allows you to tune it to your specific vehicle.

 

I found this out the hard way in a panic stop many years ago, and this was with OEM 10" drum brakes on my Tux. The rear tires came off the ground about 12" in this particular situation. Scared the crap out of me!
I told my brother what happened, and he called BS on me. He then took me Tux down the street at about 30 mph and jabbed the brakes hard. He got the rear tires off the ground about 18"!
I actually parked the Jeep a couple years after that , hardly driving it.
I used a Ford 1 ton truck pedal/master cylinder with stock Tux wheel cylinder sizes. I was running 10" rims with soft 235/75-15 tires. With weight transfer, the fronts wouldn't lock up.
-Donny

 

I mounted an adjustable proportioning valve on the firewall under my dual reservoir master cylinder. I performed a series of simulated panic stops on some back country roads, some gravel, some blacktop, and found the best setting. I have used both a single axle trailer and a tandem axle equipment trailer with brakes behind the CJ6, again performing controlled panic stops to adjust the proportioning valve. Both settings were pretty much identical, loaded or empty, once I found the "sweet spot". No swapping ends or scary brake surprises. It was the simplest and easiest solution for my application, and works effectively.