Dual master cylinder question

[1985olds]

Dumb question, but I’ve never dealt with a balance bar system before. I’m installing a Wilson’s dual master cylinder setup for the brakes on a 39 Chevy. With the balance bar adjust braking bias, there should be no need for a proportioning valve correct?

[proper]

With two master cylinders the front brake line is separate from the rear so no proportioning vale is used. Make sure the bore on the masters are sized correctly and the balance bar will do what it’s intended for.I’m also running manual dual masters with a balance bar but all from Wilwood.You will need residual valves in each line.

[Blown353]

Even with a balance bar, a proportioning valve is very helpful for street use as it introduces a "knee" in the pressure curve for the rear brakes. A balance bar solidly locks in the front to rear brake bias regardless of pedal effort or desired deceleration rates, so a balance bar setting is really only ideal for one desired braking condition and deceleration rate. On the other hand, a proportioning valve introduces a "knee" into the pressure curve of the rear brakes which can provide one front to rear brake bias below the knee point and a different front to rear bias above the knee point, which helps provide additional rear brake effort and better stopping power over a wider range of braking conditions than the balance bar by itself.

If you only have a balance bar and adjust it so the rear brakes lock up slightly after the fronts during a max effort stop (say that brake bias at a max effort stop ends up 70% front / 30% rear) that means the front to rear brake bias is always going to be 70F/30R regardless of pedal input or deceleration rate-- so the rear brakes are not doing as much work as they could actually contribute during light to moderate effort stops where there isn't much weight transfer-- and these kinds of stops are the most frequently done stops on the street. This kind of setup overworks the front brakes and underworks the rear brakes for street use. During light and moderate braking conditions without much weight transfer, the rear brakes can provide a much larger percentage of the total braking effort.

Using a proportioning valve in conjunction with a balance bar on the street allows you to adjust the balance bar to provide a higher percentage of rear braking effort during mild to moderate stopping rates often encountered on the street, while the proportioning valve will reduce the pressure to the rear brakes during a max effort stop (which has the greatest amount of weight transfer and needs less rear braking effort) to keep the back wheels from locking prematurely. It does take a bit of tuning to find the happy point for the adjustments of the balance bar and the proportioning valve for street use. With the help of a braking calculation spreadsheet and putting in all the brake data for the car, for street use I'll often start with targeting about a 58% Front / 42% Rear bias on the balance bar and then tune the rear brake behavior and lock up point during hard stops with only the prop valve. With this kind of setup, during light and moderate effort stops below the knee point of the prop valve you'll have a 58F/42R brake bias so the rear brakes are doing more work during lighter and moderate stops, but during a harder stop where brake pressure increases and weight transfer increases and you need less rear brake effort the prop valve will be above the knee point and reduce pressure to the rear brakes to prevent early brake rear lockup and say provide a 70F/30R brake bias so during a hard stop the back brakes lock up just after the fronts.

When the proportioning valve goes above the knee point, that means some of your pedal input effort is being "wasted" as some of the pounds you're putting into the pedal with your foot are being "shed" and reduced by the proportioning valve instead of going directly to the rear calipers.

For track use where all stops are maximum effort and you don't care about the rear brakes being underutilized during light stops, you can dial the proportioning valve all the way up so it doesn't come into play at all, and adjust the balance bar for the proper amount of rear braking force needed during maximum effort brake applications. Taking the proportioning valve out of the picture also ensures every pound of pedal input goes directly into the output to the brakes.

[1985olds]

Even with a balance bar, a proportioning valve is very helpful for street use as it introduces a "knee" in the pressure curve for the rear brakes. A balance bar solidly locks in the front to rear brake bias regardless of pedal effort or desired deceleration rates, so a balance bar setting is really only ideal for one desired braking condition and deceleration rate. On the other hand, a proportioning valve introduces a "knee" into the pressure curve of the rear brakes which can provide one front to rear brake bias below the knee point and a different front to rear bias above the knee point, which helps provide additional rear brake effort and better stopping power over a wider range of braking conditions than the balance bar by itself.

If you only have a balance bar and adjust it so the rear brakes lock up slightly after the fronts during a max effort stop (say that brake bias at a max effort stop ends up 70% front / 30% rear) that means the front to rear brake bias is always going to be 70F/30R regardless of pedal input or deceleration rate-- so the rear brakes are not doing as much work as they could actually contribute during light to moderate effort stops where there isn't much weight transfer-- and these kinds of stops are the most frequently done stops on the street. This kind of setup overworks the front brakes and underworks the rear brakes for street use. During light and moderate braking conditions without much weight transfer, the rear brakes can provide a much larger percentage of the total braking effort.

Using a proportioning valve in conjunction with a balance bar on the street allows you to adjust the balance bar to provide a higher percentage of rear braking effort during mild to moderate stopping rates often encountered on the street, while the proportioning valve will reduce the pressure to the rear brakes during a max effort stop (which has the greatest amount of weight transfer and needs less rear braking effort) to keep the back wheels from locking prematurely. It does take a bit of tuning to find the happy point for the adjustments of the balance bar and the proportioning valve for street use. With the help of a braking calculation spreadsheet and putting in all the brake data for the car, for street use I'll often start with targeting about a 58% Front / 42% Rear bias on the balance bar and then tune the rear brake behavior and lock up point during hard stops with only the prop valve. With this kind of setup, during light and moderate effort stops below the knee point of the prop valve you'll have a 58F/42R brake bias so the rear brakes are doing more work during lighter and moderate stops, but during a harder stop where brake pressure increases and weight transfer increases and you need less rear brake effort the prop valve will be above the knee point and reduce pressure to the rear brakes to prevent early brake rear lockup and say provide a 70F/30R brake bias so during a hard stop the back brakes lock up just after the fronts.

When the proportioning valve goes above the knee point, that means some of your pedal input effort is being "wasted" as some of the pounds you're putting into the pedal with your foot are being "shed" and reduced by the proportioning valve instead of going directly to the rear calipers.



For track use where all stops are maximum effort and you don't care about the rear brakes being underutilized during light stops, you can dial the proportioning valve all the way up so it doesn't come into play at all, and adjust the balance bar for the proper amount of rear braking force needed during maximum effort brake applications. Taking the proportioning valve out of the picture also ensures every pound of pedal input goes directly into the output to the brakes.

thank you! Looks like more research for me. Also, a prop valve will make plumbing easier, so that’s a win too!