Disc brake swap proportioning valve options

Starfury · 05-11-2008, 12:57 AM

Ok, so for everyone here who's done a front disc swap, I'd like to know if you chose to use an adjustable proportioning valve or the stock combo valve, and what your opinions are on the setup you chose. The CSRP (discbrakeswap.com) kit has an option between the two styles and I need to decide which I want to get.

I'm leaning towards the OE combo valve atm, but I'm open to input on the matter.

nassaubayman · 05-11-2008, 11:53 AM

I upgraded from manual drum to power front-disc on my '67 coupe. I didn't want to have to splice and re-flare brake lines, so I decided to see how it worked without a proportioning valve. I have ZERO problems with one end locking up before the other. Depending on how you intend to use your car, you might want to consider not using one. Mine is a weekend cruiser - never raced, but occasionally romped on down a long street.

I used original '67 disc brakes with an SSBC booster. I don't know if that would make a difference.

If I were to use a proportioning valve, SSBC makes a proportioning/distribution combo valve. That way, no splicing/re-flaing would be required.

Starfury · 05-11-2008, 01:02 PM

Wow....basically, you're not using any front braking power, and your total stopping distance is actually worse than before you did the swap. That's what happens when you don't have a proportioning valve. Disc brakes require MUCH more pressure to operate than drums do, and you have to have some way of keeping that pressureat the front calipers rather than spread through the entire braking system. I very strongly suggest you plumb in a proportioning valve. Not using one is not an option for me, nor should it be for anyone else doing the upgrade.

blue66mustang · 05-11-2008, 01:20 PM

Quote:

ORIGINAL: Starfury

Wow....basically, you're not using any front braking power, and your total stopping distance is actually worse than before you did the swap.

UM... the brakes will work just fine if he is using a dual bowl master cylinder. There are 2 SEPERATE systems this way. The front is isolated from the rear. The proprotioning valve actually stands off or lowers the pressure to the rears so they wont lock prematurelyin a panic stop before the fronts. This way the car wont snap spin on you.

So actually you SHOULD use the proportioning valve but for an entirely different reason.

rmodel65 · 05-11-2008, 01:31 PM

what tad is implying is that the disc system need about twice the operating pressure to work properly. if he did indeed not install the prop valve then he is getting insignificant pressure to the disc up front.

i went with the ssbc force 10(13 inch) and the ssbc (10.5 inch) for the rear i used prop valve i havent fiddled with ti one way or the other. since ive never driven it out of thr yard but once

Starfury · 05-11-2008, 01:39 PM

The front is not isolated from the rear.Fluid is isolated, but not pressure. Pressure remains the same in the ENTIRE braking system at all times unless you have a proportioning valve. Open up a rear bleederscrew and see how much pressure you have at the front.The m/c has two pistons, both of which are going toprovide the same pressure (unless you have a setup with one larger piston, which I doubt). Sincedrum systemsrequire less pressure than disc brakes, they're going to start functioning long before the front discs ever get a chance to do any work. The fronts will work, but not until long after the rears start working.

The front brakes are supposed to provide the majority of your stopping power. Not using a proportioning valve in a disc/drum systemtries to movethe primary stopping power to the rear brakes. Not only does it completely negate the point of doing a front disc conversion, but it's not safe.

nassaubayman · 05-11-2008, 07:26 PM

Sorry you don't believe me, but my car stops just fine. Don't believe me? I'll have another member, JamesTopless67, who drove my car both before and after the upgrade, chime in and confirm they work great.

There's a guy on ebay named Chockostang, who specializes in factory original Mustang brakes and steering. Call him and he'll tell you the same thing - the proportioning valve isn't needed. 217-882-2083

I wasn't sure I wanted to believe him either. But I thought it wouldn't hurt to take it to an open road and stomp on the brakes. Sure enough, they stopped just fine. Otherwise, I would have gone straight home and installed the proporting valve.

blue66mustang's assessment is correct. However, I did stomp them and get them to lock up. No snap spinning occurred. My car stopped straight. I did the brake upgrade a year ago and I've stopped hard more than a couple times since then. I've never had even a hint of trouble.

SalikDDD · 05-11-2008, 07:45 PM

i never used the combo valve. i used a t fitting to split the front brakes and then i ran a single line to the back and split them at the axle. also installed the valve that came with the kit for front and rear adjustment.

Starfury · 05-11-2008, 07:54 PM

It's not a matter of belief, it's a matter of physics. Blue66mustang's assessment is not correct at all. If you understand howthe braking system works, you understand why a proportioning valve is necessary. If it wasn't needed, manufacturers wouldn't put them on every single car with a disc/drum setup. Auto manufacturers are not in the business of wasting money.

You may not have any noticable issues seeing as you don't drive your car hard or often, but that doesn't mean the braking system is functioning properly. You're still using the rear drums to stop the car much more than the front pads. It may stop ok for you in a straight line, but try stomping on the brakes in a wide corner. I bet the car gets real tail happy.

The facts:
Disc brakes require more force to stop than drum brakes.
Without a proportioning valve, hydraulic pressure in a braking system remains constant at all points within the system, no matter whichhalf of the m/c you're working off of.
If you apply the same amount of force to drum brakes as to disc brakes, the drum brakes will be trying harder to stop the car.

blue66mustang · 05-11-2008, 11:13 PM

Pleae re read my OP.

Metering Valve
The metering valve is designed to equalize braking action at each wheel during light brake applications. A metering valve is used on vehicles with front disc brakes and rear drum brakes and is located in the line to the disc brakes. The metering valve functions by preventing the disc brakes from applying until approximately 75 to 135 psi has built up in the system.
Proportioning Valve
The proportioning valve also equalizes braking action with front disc brakes and rear drum brakes. It is located in the brake line to the rear brakes. The function of the proportioning valve is to limit pressure to the rear brakes when high pressure is required to apply the front disc. This prevents rear wheel lockup and skidding during heavy brake applications.
Combination Valve
The combination valve (fig. 7-20) combines several valve functions into a single assembly. It functions as a-
Metering valve- holds off front disc braking until the rear drum brakes make contact with the drums.
Proportioning valve- improves front to rear brake balance at high deceleration by reducing rear brake pressure to delay rear wheel skid.