Adjustable Proportional Valve - Do I need one?

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Haney

Haney

www.carsonandironmt.com
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I just completed my install of front/rear suspension and brakes on my 72 Dart and I am getting ready to bleed and align. Before I bleed, I want to ask a question: Do I need an adjustable proportional valve in the main rear brake line feed? I installed Dr. Diffs 11.75 front disk kit and a Right Stuff Rear Disk kit which is based on the GM screw type calipers. I also installed a late model aluminum master cylinder kit (1 1/32 bore). Car has manual brakes.
 
You may or may not need a proportioning valve. Bleed the system & take it out for a spin to an area where you won't get into trouble to try out the brakes.
 
I agree. There's no way to know until you take it for a drive and see how the brakes react. That 1 1/32 bore master has me concerned a little though. Most people that convert to 4 wheel discs use the 15/16 bore master. The 1 1/32 might give you a rock hard pedal and poor braking.
 
My 67 Barracuda was upgraded to the KH disc brakes, it was determined that it didn't need the proportional valve.


Alan
 
Easy way to tell is find a dirt/gravel road and with a helper/spotter lock up brakes and
see if back brakes lock too early. Sure you will find a dirt/gravel in Tenn. if not a wet road.
 
Disc/drum and disc/disc are completely different situations. The factory never made a disc/disc system, so you are on your own. Modern disc/disc systems do not use proportioning valves — they control the amount of braking front to rear by balancing the overall stopping power — the size of the rotors and the number of pistons in the calipers. The factory proportioning valves were designed for disc/drum systems, and they do not control the total amount of braking force at the rear brakes — what they do is modulate the rate at which the braking force increases due to brake pedal pressure, to compensate for the different response characteristics of drum brakes versus disc brakes. Disc brakes respond linearly; drum brakes do not — as more shoe surface contacts the drum, it pulls itself tighter, creating an increasing, non-linear braking effect. This is what the proportioning valve compensates for, by tapering off the rate of line pressure increase.

What this means for your unique disc/disc system: you will have to test it on the road to evaluate its front/rear balance. Since the rear probably has smaller rotors and calipers (you didn't describe them), it may work well as is. If the rears lock up prematurely, than an aftermarket adjustable proportioning valve (such as Summit Racing's) should be installed in the rear brake line. This will let you adjust the line pressure to modulate the rear brake response.

Tip: you do not need to cut and flare brake lines to install the PV. Just buy a short pre-made length of brake line at your local auto supply, and bend it into a short loop. Then you can undo the rear brake hard line from the combo block and route that to the PV, then insert the loop between the PV and the combo block. This locates the PV down low near the combo block — a lot of people think they have to locate it up high in the engine compartment where they can easily reach the adjustment knob, but this is unnecessary — you will only need to adjust it a few times over the first few rides. After that it will not need further adjustment.

To 66jim: The first test is whether the rear brakes lock up in a panic stop on dry pavement. Once that is adjusted out, then a wet test may make sense, if a safe location can be found.
 
if possible, you should also have the tires you plan on running.
example: i've got my disks installed but know i'm going to 18" wheels and wider tires so the brakes are going to behave way different later
 
If the car sometimes has slicks on it, you may want more pressure to the rear at the track.

I had factory front discs on the front of a Challenger years ago. It had 11" rear drums, not the 10" that factory disc/drum cars had. It would lock the rears with street tires, but stopped great with slicks.
 
mvh said:
Disc/drum and disc/disc are completely different situations. The factory never made a disc/disc system, so you are on your own. Modern disc/disc systems do not use proportioning valves — they control the amount of braking front to rear by balancing the overall stopping power — the size of the rotors and the number of pistons in the calipers. The factory proportioning valves were designed for disc/drum systems, and they do not control the total amount of braking force at the rear brakes — what they do is modulate the rate at which the braking force increases due to brake pedal pressure, to compensate for the different response characteristics of drum brakes versus disc brakes. Disc brakes respond linearly; drum brakes do not — as more shoe surface contacts the drum, it pulls itself tighter, creating an increasing, non-linear braking effect. This is what the proportioning valve compensates for, by tapering off the rate of line pressure increase.

What this means for your unique disc/disc system: you will have to test it on the road to evaluate its front/rear balance. Since the rear probably has smaller rotors and calipers (you didn't describe them), it may work well as is. If the rears lock up prematurely, than an aftermarket adjustable proportioning valve (such as Summit Racing's) should be installed in the rear brake line. This will let you adjust the line pressure to modulate the rear brake response.

Tip: you do not need to cut and flare brake lines to install the PV. Just buy a short pre-made length of brake line at your local auto supply, and bend it into a short loop. Then you can undo the rear brake hard line from the combo block and route that to the PV, then insert the loop between the PV and the combo block. This locates the PV down low near the combo block — a lot of people think they have to locate it up high in the engine compartment where they can easily reach the adjustment knob, but this is unnecessary — you will only need to adjust it a few times over the first few rides. After that it will not need further adjustment.

To 66jim: The first test is whether the rear brakes lock up in a panic stop on dry pavement. Once that is adjusted out, then a wet test may make sense, if a safe location can be found.
Click to expand...
Basically agree.
Unfortunately the situations where proportioning valve is needed is dangerous to test.
The job of the valve is to allow the same line pressure to all 4 corners from initial application to moderate increase in pedal force.
As the car's weight shifts forward, if yet more braking is required (over 400 or 500 psi - you can look it up) the rear drums get less of an increase than the front.
This is especially critical with duo-servo drums. Why? Because the mechanics of these drum systems leverage the action of the shoes. Its usually called self-energizing or something similar. Leveraging the stopping power on the rear (which already is losing down force on the wheels) can cause lockup. Rear lockup means the front of the car is stopping and the rear is not.
You can test in steps as mvh suggested.
With disk/drum setup, I always recomend erring on the safe side and at least go with the factory prop valve.
 
Last edited:
Haney said:
I just completed my install of front/rear suspension and brakes on my 72 Dart and I am getting ready to bleed and align. Before I bleed, I want to ask a question: Do I need an adjustable proportional valve in the main rear brake line feed? I installed Dr. Diffs 11.75 front disk kit and a Right Stuff Rear Disk kit which is based on the GM screw type calipers. I also installed a late model aluminum master cylinder kit (1 1/32 bore). Car has manual brakes.
Click to expand...

I dont run one on mine , just the orig. feed block for the old drum brakes .
Your master cyl. piston is too big -------------jmo
 
I have Wilwood disc on all 4 corners of my bracket car (71 Demon). It doesn't need a proportioning valve as my car only weighs 2500 lbs & I have no problems making the first or second turn out at most track.
 
7/8" or 15/16" bore MC is probably what you need. My bracket car with Wilwood 4 wheel disc had a 7/8" bore.
 
"That 1 1/32 bore master has me concerned a little though. Most people that convert to 4 wheel discs use the 15/16 bore master. The 1 1/32 might give you a rock hard pedal and poor braking."

Doctor Diff sells a 15/16 bore master. His masters have the extra recess in the back of the piston to lock the pushrod in place. If you get one from a parts store, the back of the piston won't be machined to lock the pushrod.
 
Dr. Diff is where the one I have came from..... He recommended the one I have and his site says the larger bore will work?
 
Cuda Al said:
My 67 Barracuda was upgraded to the KH disc brakes, it was determined that it didn't need the proportional valve.


Alan
Click to expand...
............................

Is yours front disc / rear drum ?
If so...
How did you determine a prop valve was not needed ?
 
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