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.
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.
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 pressure at 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.
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 prematurely in 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.
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Joined: 10/10/2005 From: brunstank ga Status: offline
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
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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 bleeder screw and see how much pressure you have at the front. The m/c has two pistons, both of which are going to provide the same pressure (unless you have a setup with one larger piston, which I doubt). Since drum systems require 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 system tries to move the 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.
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.
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.
It's not a matter of belief, it's a matter of physics. Blue66mustang's assessment is not correct at all. If you understand how the 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 which half 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.
< Message edited by Starfury -- 5/11/2008 4:57:33 PM >
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.
< Message edited by blue66mustang -- 5/11/2008 8:16:54 PM >
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.
The combo valve is a plumb and forget valve while the adjustable one requires a bit of tuning. I'd use the combo type for street use.
I agree with your assessemnt that a proportioning valve is necessary. There are vendors that claim they are not needed. Then again, these same vendors sell "complete" kits that don't include master cylinders, hoses, proportioning valves, or dust shields.
Specializing in front disk brake swaps for those models of cars introduced in the 1960's that I may not mention or the company that made them will sue me.
Thanks, Degins:) I don't mind a bit of tuning, but the car is a long way from being track-ready which is why I'm leaning towards the combo valve. Mostly I'm curious as to how well it works compared to an adjustable unit. Obviously the adjustable unit will allow you to fine tune the brake bias to whatever you want, but how is the brake bias with the combo valve?
ORIGINAL: Starfury 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.
The front is isolated from the rear, that is the reason for a split system, as mandated by law. If I allow air into the rear circuit it does not effect the front circuit and vice versa.
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ORIGINAL: Starfury Open up a rear bleeder screw and see how much pressure you have at the front. The m/c has two pistons, both of which are going to provide the same pressure (unless you have a setup with one larger piston, which I doubt). Since drum systems require 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.
With the bleeder open you cannot have any pressure, partially closed maybe some pressure and closed, up to a thousand pounds of pressure. The rear drums require much more fluid than do disks. So your assertion is simply wrong. Because even though the same volume of fluid ( not pressure ) is moved in both circuits, the fluid moves into a larger area in the rear circuit.
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ORIGINAL: Starfury The front brakes are supposed to provide the majority of your stopping power. Not using a proportioning valve in a disc/drum system tries to move the 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.
Wrong as pointed out above. It will vary from brake system to system, depending on components and condition. It is not something that can be issued as a blanket statement.
I recommend testing the brake bias in a wet empty car park or empty dead end street with little in the way of obstacles. If you car pulls up straight and without either end locking up before the other, then I would not change the system. Usually I would check emergency braking and also check the lock up point for front and back.
Thanks, Degins:) I don't mind a bit of tuning, but the car is a long way from being track-ready which is why I'm leaning towards the combo valve. Mostly I'm curious as to how well it works compared to an adjustable unit. Obviously the adjustable unit will allow you to fine tune the brake bias to whatever you want, but how is the brake bias with the combo valve?
What makes you think a combo unit isn't adjustable?
I think the upside to a combo unit far outweigh having a separate proportioning valve. You have less splices and flares to deal with. You have one less piece to worry about where to mount. The adjustment knob is in a handy spot. The only down side is it costs more.
The CSRP kit comes with either a separate adjustable proportioning valve or the OE style combo valve. To my knowledge, SSBC is the only company that makes an adjustable combo valve like that.
The front is isolated from the rear, that is the reason for a split system, as mandated by law. If I allow air into the rear circuit it does not effect the front circuit and vice versa.
With the bleeder open you cannot have any pressure, partially closed maybe some pressure and closed, up to a thousand pounds of pressure. The rear drums require much more fluid than do disks. So your assertion is simply wrong. Because even though the same volume of fluid ( not pressure ) is moved in both circuits, the fluid moves into a larger area in the rear circuit.
Wrong as pointed out above. It will vary from brake system to system, depending on components and condition. It is not something that can be issued as a blanket statement.
I was trying to avoid giving a brake mechanics lesson here, but apparently it's necessary.
First of all, the FLUID is isolated between the front and rear braking systems, pressure is not. There are two pistons in the m/c. They are connected hydraulically, not mechanically. The pedal rod pushes on the rear piston which builds pressure in the front circuit which then pushes on the front piston to build pressure in the rear circuit. It's still a single closed fluid circuit. If you lose pressure in either circuit, you're losing pressure everywhere. Pascal's law: fluid pressure in a closed system remains the same at all points within the system. This is why the pedal drops to the floor if you open one bleeder screw. The drop in pressure is spread across the entire system.
If the two systems were completely isolated, you'd never know if you had a problem with either system until you noticed a drop in braking performance. The pedal would remain the same as long as one one system was working properly. Obviously this would be unsafe, which is why braking systems are not designed this way.
Second, wheel cylinders requiring more fluid doesn't change the fact that they require less pressure. This is why there's a metering valve, to make sure you have some risidual pressure in the rear to keep the shoes close to the drums so you don't have the fronts contacting the discs long before the shoes touch the drums. You're not building much pressure anywhere until both pads and shoes are contacting their respective friction surfaces. And like I explained, pressure is constant throughout the entire system, so without some form of proportioning valve and metering valve, the drums are doing far more work as pressure builds than the discs are.
Third, none of what I said is incorrect. I generally try to be fairly nice to people here and give them a chance to understand why they're wrong, but you're all making it very difficult. A lot of engineering goes into designing a vehicle's braking system. If you don't understand in detail how a hydraulic braking system works, you shouldn't be modifying your braking system outside of the factory design.
A combo valve is exactly that, a combination of distribution block and/or prop valve and/or metering valve. The CSRP combo valve is a repro of the OE valve. It looks similar to the 4-wheel drum distribution block, which looks like a block of brass. It's a non-adjustable unit, so the brake bias is set at factory spec.
The other option they offer is an adjustable in-line proportioning valve/metering valve combo which gets plumbed in-line with the rear brakes. It allows you the ability to fine tune the brakes but it can be more of a hassle because you do have to tune it, and as you said that's one more splice you have to deal with.
That SSBC valve is nice, but like you said, it's more expensive, and it may be overkill for me right now. However, what I might do is just get the OE style valve for now and consider upgrading to the SSBC valve later if I decide I need it. That's a lot easier than plumbing an in-line adjustable valve down the road.
Tad, I have a question for you. I always thought that the reason for the duel bowl mc is that the front and rear were isolated making them safer than the "fruit jar" single bown mc. But I have also noticed that when bleeding brakes and opening the bleeder screw the pedal goes straight to the floor, thus loosing all braking. Why then is the duel bowl mc safer? Kip
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Disc brake swap proportioning valve options - 
