How do you know if your prop valve is set correctly?

[dolf]

Is there a way to tell if you're getting the most form the rear brakes? What exactly does the proportioning valve do?

[Hydratech®]

This information is available on our website: http://www.hydratechbraking.com/braketech2.html

Make sure to also read the StopTech link thoroughly too.

[jpgolf14]

Is there a way to tell if you're getting the most form the rear brakes? What exactly does the proportioning valve do?

Hydratech's article is solid on the how to adjust.

For you second question. The prop valves controls the pressure of the brake fluid to your rear brakes as a percentage to front brake pressure. Basically, the idea is to to tune the system so you utilize all the stopping potential (grip) of your tires. The amount of force the tire can resist is linearly dependent on the weight on the tire, and the coefficient of friction of the tread on the road. Since, you likely have the same tires on front and rear the only important factor is the weight on the wheels. Obviously most of our cars have more weight on the front of the car than rear and this balance gets worse as you decelerate (weight shift forward). So based on weight on tire and the efficiency of the front and rear braking systems (rotor diameter and piston area) the rear brakes typically need to see far less pressure than the fronts.

For the sake of argument lets assume the rear brakes need 50% of the pressure of the fronts.

Now, we only test the tires grip on a max performance stop, right? On most normal stops, we are not even close using all the tire's potential. In this case more rear bias would be great and result in a shorter stop. As long as the rear brake pressure does not cause the rear wheel to lock up, we will take as much rear bias as possible. For this reason the prop valve is designed to meter the rear pressure based on front pressure. In other words, up to a certain front pressure, the rears are getting 100% brake pressure, then at the knee point the rear pressure starts trailing away. On an adjustable prop valve, the knee point is that you are adjusting. You are moving it right or left.

Here is a chart for a tilton prop valve with distinct knob positions. Notice how they control when the rear fluid starts trailing off. Up to that point the rear pressure is the same as the front pressure.

https://static1.pt-content.com/images/pt/2012/09/PropValveChart-1.jpg

[MIKE67]

To the original question, the main thing that you are trying to prevent is the rear brakes locking up first. This will cause the back of the car to swap ends with the front - NOT fun!!

[dolf]

Thanks, so I am assuming the control knob + or - means pressure, correct? If I am not locking the rears then I can keep increasing pressure to get more bite from the rears.

[Hydratech®]

People get this wrong everyday, so make sure you get this right:


SCREWING THE KNOB INWARD INCREASES PRESSURE TO THE REAR BRAKES

UNSCREWING THE KNOB OUTWARD DECREASES THE PRESSURE TO THE REAR BRAKES


Here are the Summit instructions - applies to all of these types of knob adjustable design inline style designs:

http://static.summitracing.com/global/images/instructions/sum-g3905.pdf

:6gears:

[keithq69]

Be sure to try it on a wet surface under a controlled situation before you rely on your adjustment in the rain.
You're most likely to lock the rear under heavy braking and wet conditions.

[Skip Fix]

Be sure to try it on a wet surface under a controlled situation before you rely on your adjustment in the rain.
You're most likely to lock the rear under heavy braking and wet conditions.

That or find a loose gravel road. Front spring rate affects how the rear weight bias moves to the front also so heavy braking shifts it to the front compared to lighter or more moderate braking. Stiff front springs do not do this as much as soft spring/big sway bar setups do.

My 78 TA's factory front springs and factory prop valve would allow the rear drums to lock up in a hard braking turn.