1. Brake & Wheel Systems
2. Proportioning Valve Options

When changing from rear drum brakes to rear disc brakes it is important to pay attention to the proportioning valve that is used.

Factory prop valves are setup for the factory diagonal braking system. The FR/RL and FL/RR brakes share banks. This way if a bank fails it is easier to control the car with one diagonal bank still functioning.


Here is a chart that I created showing common proportioning valves and their slopes with crack points (I also added some custom options).


The graph shows the following:

The rear pressure is the dependent variable (Y axis) based on the front pressure which is the independent variable (X axis).

The rear pressure rises 1:1 with the front until the crack point. At which time the rear changes its slope specific to the slope in the valve.

So as you put your foot on the brake the pressures rise 1:1 until the crack point. At which time the rear starts to get proportionally less pressure than the front as more pressure is applied to the pedal.


Step1:

If your rear locks up early then your crack point is probably too high. If your rear locks up late then your slope is probably too large.

Step 2:

If you lock up with medium application and have a high slope valve then the slope is probably too high. If you lock up in the middle with a low slope valve then your crack point is probably too high.

Changing the slope and crack point both adjust things depending on what you want and when in the braking cycle you want more/less pressure.

Comparing the 400/.27 and 300/.43 lines shows a good example of fine tuning... The valves provide similar pressures and aren't radically different but tune the max pressure to the rears differently through the range. The white valve (400/.27) has more initial pressure than the tan valve but the tan valve (300/.43) has more max pressure. They have the same pressure at around 750psi.

They changed the Spirit valve down to a 300/.27 for some models. This implies that they had early tire lockup with the 400/.27 valve and late rear lockup with the 300/.43 valve on those models. Of course they ended up with quite a bit lower late pressure when doing that. Perhaps a 250/.43 would have worked better. That would require a spring that they didn't have but we could try to source one that would do that.


Custom valves that were not offered from the factory can be made with some simple changes or mixing and matching parts from two different valves:

There are the following options with our prop valves:

Crack Point (Crossovers):

300/400/500/600/750/800 (with 300/400/500 the only real ones we care about)

Slopes:

.27
.43
.59

The springs do the crossover and there must be an orafice that does the slope.

I am pretty sure that if you take a the spring from a yellow tag prop valve (400psi) and swap it into an lbody prop valve (.27 slope) you will get a white valve.... If you started with a 300/.43 and an lbody valve you could make the 300/.27

Once again this is something you have to be very careful doing. If modified, valves should be pressure tested after the mod is done to verify that it works. Additionally brakes should be tested in an off road environment before driving on the street.


Notes on aftermarket adjustable proportioning valves:

Aftermarket valves assume non diagonal systems so in order to properly use an aftermarket adjustable prop valve the brake lines should be reconfigured.
Also note that most aftermarket prop valves have fixed slopes (almost always .57) and only the crack points are adjusted. The slopes are commonly not optimal for FWD cars.