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Discussion Starter #1
I have a Mustang II IFS from Rod & Custom Motorsports w/11" discs, which uses a giant GM master cylinder. The master cylinder has no built-in front-rear brake bias. I certainly don't recall installing a proportioning valve. I spoke w/R&C and verified that there is no built in bias to the MC, and he added, "I'm surprised you don't need one now for the disc/drum combo". As it is now, I can stop very smoothly with the front discs and rear 10" drums, not premature lock up of the rear, etc.

Maybe I do have a proportioning valve some where but don't know where, or is it possible that I'm running without one just fine?

I'm going to get 11" Explorer discs for my new 9" rear end, and I don't know if I'll need a proportioning valve of some kind. If I'm doing okay with the 10" drums, will the 11" discs require the proportioning valve? I'm baffled. Thanks.
 

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i woudl think u dont have a proportioning valve and ur not getting any rear wheel lock up that ur not gettin any rear brakes at all...but as far as for switching to explorer discs in the rear you will not need a prop valve
 

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Discussion Starter #3
Thanks Aaron. Before getting the front discs, when I had 4 drums, stopping was scary, but it stopped. I'm baffled about what I have now. I'll have to check it out now. The car stops very well with the power discs, and the rear drums. I recently installed an E-brake which doesn't work worth a damn, so maybe somehow a lack of a p-valve is the culprit (although I think the cable entirely bypasses any hydraulic operation). I just might have to get a Wilwood prop valve. Thanks again.
 

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Youre right about the parking brake bypassing the hydraulic system all together. Most OEM proportioning valves are Blocks of brass that split 2 incoming brake lines into 3-2 for the front and 1 that runs to the back. (Some MC's have 3 lines, 2 of them go to the front, 1 to the back)
The rear portion is seperated from the front and also has a dampnig effect on the line pesssure, making sure you have the right "proportion" of pressure and/or volume between the front and rear. The front half of the valve is simply a "T" splitter in disc brake systems. If you dont have one of these, then your system will be out of balance and wont work to its potential. The best way to go would be to put an adjustable valve in the rear line going to the rear brakes, and adjust it accordingly.
 

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also, when you put the explorer brakes on there, make sure that you use a master cylinder that does not have a pressure check valve on the rear brake side of it.
 

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Discussion Starter #6
Great info Chico. I checked and found that I don't currently have a proportioning valve, but the difference between the stopping power of the front discs which do most of the braking vs. the inefficiency of the rear drums, combined with how light my vehicle is, has simply allowed me to get away without the p valve. I'm going to get a Wilwood adjustable P valve for the job. You're right. I'm sure I'm currently not getting the performance I should be getting from my current disc/drum set up, but the rear discs will demand it. Thanks.
 

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I don't know what kind of car you have but my Mustang has the p-valve in the T at the rear end, where the rubber brake hose splits to the left and right brake lines.
 

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On my Rod & Custom front i put a Wilwood on each of the front and back systems. I also put a 10 lb. residual valve on the rear otherwise you get brake fluid trying to come back through the master cylinder when you let off the brakes. You are supposed to run a 2 lb. residual on the front but I didn't do that.
 
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