Proportioning valve for rear discs

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DogTownDart

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How’s it going everyone, I have a 70 dart with factory front disc and drum in the rear, I will be purchasing a rear disc conversion kit soon from pirate Jack and was hoping to get some insight. When I purchase the kit do I also need to purchase a new proportioning valve and or new master cylinder? Any info on this would be helpful, thanks!
 
The proportioning valve will be just fine.

The master cylinder has a residual pressure valve behind the seat of the forward most port (rear brakes) that will be needed to be removed.

An easy out should be able to remove the seat without damaging it. But replacements are available.
 
Have you tried contacting @DoctorDiff about his rear brake kits? I know he offers his front disk kits in the 5x4" pattern as well, he may do the same for some of his rear disk kits. You'll get far better parts and service from DoctorDiff than from PirateJack, if he sells something that's an option for you. His prices may be a bit higher, but so is the quality of parts you'll receive.

As for the rest of it, you master cylinder will work with the residual pressure valve removed but you may find you want a different bore master cylinder to better match the pressure and pedal requirements of the new brakes. Same for the proportioning valve- the stock valve should work, but, you may have better results adding an adjustable proportioning valve to better match the new braking capabilities.
 
Whether you need a new prop valve will depend on whether the piston size in the new rear calipers have been sized to work with the existing prop valve.
 
No...
A 4 wheel disc brake system does NOT use a proportioning valve unless the bias is out of whack. They can use a distribution block that has no proportioning to it. Those are found in 4 wheel drum brake cars where they were designed with specific wheel cylinder sizes front vs rear to achieve natural proportioning. Again, 4 wheel drum as well as 4 wheel disc cars do not need "proportioning" if the system is in balance.
2 to 1. That is the bias that is ideal. You want twice the braking force at the front compared to what you have at the rear. For example if the rear caliper piston is a single 1.5" unit, the front needs to be double the surface area. To make the math easier, you'd need a 2 piston front caliper with dual 1.5" pistons. Doing it backwards, find out what the surface area is of your stock front brakes and choose a rear brake kit with a caliper piston size that is within the 2 to 1 ratio.
If you have a situation where the front to rear bias is not close to 2 to 1, the crutch is an adjustable proportioning valve. A fixed/NON adjustable valve will not work. Traditionally, "proportioning" valves were used in front disc/rear drum systems to reduce pressure to the rear since the drum brake system has the self energizing feature and also because the fluid/pressure demands of the front calipers are far greater than the rear drum wheel cylinders.
A front disc/rear drum system without a proportioning valve would always result in rear wheel lockup.
A four wheel disc setup with a non adjustable proportioning valve results in the rear brakes never getting enough pressure.
 
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Subscribed, I have the P Jack SBP rear disc kit , not installed yet, for my 70 Dart with KH front discs.

I am going to get the Concentric one from Rock Auto, looks very close to stock. Advantages, looks stock, same fittings as stock, ready to bolt on, price. I'll add an adjustable prop valve on the rear line.

If you're not worried about stock appearance, then I would go with the Dr Diff, as previously mentioned. It comes with an adapter to bolt it up, about $90

What I wasn't told by PJ was that the rear disc kit will NOT work with 14 wheels. The directions in the kit stated this. So...I'm saving to order a full set of Cragar SS, 15x7, SBP, from Summit. Last time I checked they were about $250 each

I have had no luck contacting PJ after the sale.

Good luck.
 
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I put 4 wheel disk on a 56 Ford Pickup.

The fronts were single piston (don't recall the size) Granada? maybe.

Rears were Versailles rear with built in e brake, (such a great setup,)

I added an adjustable valve and had to set it almost all the way tight., (10 to 1) to keep the rear from locking up.

Rear tires were 295 50 15

Fronts were 255 50 15

The factory valve on our a bodies will be a good place to start, if you need anything else adjustable is the ticket.

If you can size the fronts and rears great, but typically the options are limited to what's available.
 
Have you tried contacting @DoctorDiff about his rear brake kits? I know he offers his front disk kits in the 5x4" pattern as well, he may do the same for some of his rear disk kits. You'll get far better parts and service from DoctorDiff than from PirateJack, if he sells something that's an option for you. His prices may be a bit higher, but so is the quality of parts you'll receive.

As for the rest of it, you master cylinder will work with the residual pressure valve removed but you may find you want a different bore master cylinder to better match the pressure and pedal requirements of the new brakes. Same for the proportioning valve- the stock valve should work, but, you may have better results adding an adjustable proportioning valve to better match the new braking capabilities.

X-2 on Dr Diff.

Great personal service from Cass, high quality parts, very knowledgeable, fast shipping.
 
Dr Diff all the way and I run discs all around and no proportioning valve of any kind, just the drum/drum block Kern referred too.
 
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Dr Diff all the way and I run discs all around and no proportioning valve of any kind, just the drum/drum block Kern revered too.
I have the same drum/drum MC now.
I was told in another thread that the MC doesn't hold enough fluid for the entire system when running 4 wheel discs

PXL_20230521_151400208.jpg
 
I have the same drum/drum MC now.
I was told in another thread that the MC doesn't hold enough fluid for the entire system when running 4 wheel discs

You'll need a Disc/drum MC then.

My car is basically a rolling Dr. Diff catalog when it comes to brakes.
 
IMO Leave the rear alone. Rear wheel disc conversions gain you little in stopping power and if you check the forums on rear disc conversions, you will find common issues like low soft pedals etc...
Upgrade the quality of the brake parts you have for better performance.
It is like upgrading from bias ply to Radial tires.
 
IMO Leave the rear alone. Rear wheel disc conversions gain you little in stopping power and if you check the forums on rear disc conversions, you will find common issues like low soft pedals etc...
Upgrade the quality of the brake parts you have for better performance.
It is like upgrading from bias ply to Radial tires.

Huh? Bias ply tires are hot garbage for street use. Radials, even the all-season hockey pucks people like to use on these cars for the "look" are a significant upgrade over the OE bias ply tires in pretty much any metric worth considering.

As far as rear disk upgrades, they can make a significant improvement. Mopar muscle did a rear disk conversion on a '73 Dart Sport and checked the stopping distances between the rear drums and rear disks from 60-0. Their result was that from 60 mph factory disks up front and factory drums in the back the car took 133 feet, 6 inches to stop. After the rear disk conversion, the stoping distance improved to 122 feet 4 inches. It wasn't a super scientific way to test it, but it was better than most of the documentation out there. And the car involved was set up the same way as a lot of these cars, much larger rear wheels than fronts.

The online article is a total mess now, probably something with being converted over or moved to Hot Rod when MM was bought out. The final distance used to be a caption on the second to last picture, but I don't see the captions popping up anymore. The hardcopy article is easier to follow, but that's the way it goes.

Rear Disc Brakes - All Bound Up - Mopar Muscle Magazine

A low soft pedal is an indication of an improper install, or mismatched parts. Like adding anything aftermarket to these (or any) cars, you can't just assume that because someone sells it and it costs a lot that it's also the right component for the car.

As for the prop valve, you do need one. The factory systems that don't use one were all sized taking into account the swept friction areas, piston bores, weight distribution and even tire sizes used front and rear. Since rear disk brakes for these cars are all aftermarket items that weren't originally designed for this application, I can pretty much guarantee that running without a prop valve will not give you balanced braking. There are far too many variables on how people set these cars up to just run a straight distribution block and expect that to work. That is not to say a factory disk/drum prop valve will give balanced braking either, an adjustable valve of some kind is pretty much necessary for most set ups. Unless you're going to bust out the physics book and start doing all the calculations to size the pistons, swept areas, CG, etc.
 
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For what is costs, while redoing your stuff... a rear-line valve is a good investment. It's especially helpful with big and little tires, like on a dragcar.
 
Not a line lock ( that is to lock the fronts ) A front/rear adjustable proportioning valve to regulate front/rear bias.
FRom bias To Radial. Massive improvement in braking as well. After all it is the tire that grips the pavement not the brakes.
Mismatched components. Of course, they never came with rear disc so you almost can't help to parts not matching. Most kits use rear GM calipers.
I would say if you really want the "look" of rear disc and more overall brake performance, go with an aftermarket complete matched brake system. Wilwood, Baer etc...
 
Not a line lock ( that is to lock the fronts ) A front/rear adjustable proportioning valve to regulate front/rear bias.
FRom bias To Radial. Massive improvement in braking as well. After all it is the tire that grips the pavement not the brakes.
Mismatched components. Of course, they never came with rear disc so you almost can't help to parts not matching. Most kits use rear GM calipers.
I would say if you really want the "look" of rear disc and more overall brake performance, go with an aftermarket complete matched brake system. Wilwood, Baer etc...

Right, but if you look at the article I linked, all they did was change the rear brakes from drums to disks. It already had radials.

Going with a complete aftermarket system is absolutely no guarantee you will have components that work well together, just that the logo will match. That's not the same as matching your components to work with each other. You're assuming Wilwood is actually engineering the entire system, and not just grabbing whatever calipers they have handy and are easiest to make fit on a Mopar. I wouldn't make the assumption that Wilwood especially does any engineering to match the components of their front and rear brake kits, since they pretty much offer the same brakes for everyone.

On the other hand, if you actually look at what you're buying you can get front and rear systems that actually were intended to work together- I run DoctorDiff front and rear disks, my front disks are 13" rotors with Mustang Cobra PBR calipers, and my rear disks are 11.7" and yep, you guessed it, Mustang Cobra rear calipers. I still use an adjustable proportioning valve, because after all my Duster is not a Mustang, runs staggered tire sizes, etc. And my prop valve is not wide open, so, there was some adjustment that was necessary.

But yeah, you can absolutely put together a combination of parts that will not work well together just buying aftermarket stuff and slapping it on, even if you buy it all from the same company. That goes for pretty much everything, not just brakes. Blaming a soft pedal just on the fact that rear disks are used is silly, it's not because the rear brakes are disks, it's because there's an issue with the combination of parts used. Pick the right components and you'll absolutely get better braking than what the factory supplied. Pick the wrong components and things might not work as they should.
 
Sorry, I was my understanding was they didn't use factory stock components but manufactured their own multi piston calipers and rotors, Master cylinders etc...Like Baer, Brembo etc.
Not conversion brackets to run other manufacturers brake components.
I would guess most people are not brake educated and just rely on what someone else who is not also brake educated for recommendations.
You must admit that most questions on any subject on these type of sites are from people who have not done their own research and would rather let someone else do that work for them.
Then they complain because they are having problems and don't understand why.
Just because you tell somebody something doesn't mean they understand what you just said.
 
Thank you all for the input, only reason I’m even changing my rear brakes is due to the passenger side rear drum locking up/seizing somehow on the highway, was cruising along and everything felt normal until someone shouted that my wheel was glowing red hot..so if I’m going to be messing around with rear brakes, might as well upgrade to discs. As far as the 14” wheel situation, I read that stock rallye’s will clear the calipers, if they don’t I guess I’ll have to whip the grinder out
 
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