Joined: Wed Aug 11 2010, 10:15AM
Location: E WA
Posts: 1230
I will be installing 1973 front discs and a Summit Racing rear disc conversion into my 1967 Polara Convertible. I am trying to figure out the proportioning valve. I believe that the stock proportioning valve will not give the rear discs enough pressure. There is a proportioning valve specifically for all disc conversions: http://www.summitracing.com/parts/RSD-PV72/ but it is not adjustable.
Howevre it appears that this valve would be installed into the rear brake line and can only reduce the pressure at that point. If I am already not getting enough pressure to the rear brakes I do not see how this helps?
What proportioning valves have any of you used with a four wheel disc conversion?
Joined: Sat Aug 19 2006, 05:03PM
Location: Ontario, Canada
Posts: 2919
I haven't done a rear disc conversion, but the principles regarding why you need a prop valve should be the same as a disc/drum system.
Any prop valve cannot increase brake pressure, only reduces it. You're not going to get any more than 100% of applied brake pressure to both front and rear brakes. Typically you want to reduce the pressure to the rear brakes by some amount. Because of weight transfer to the front of the vehicle while braking, there is less weight over the rear wheels so they may want to lock-up with less braking pressure than the front wheels. The prop valve plumbed into the rear brake line compensates for this.
If you're using the 1973 master cylinder, don't forget that there will be a residual pressure valve in the outlet for the rear circuit that must be removed to use it with rear discs. <span class='smallblacktext'>[ Edited Wed May 11 2011, 07:04AM ]</span>
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
I have factory installed front disc and a TSM kit on the rear. The adjustable proportioning valve is the one shown here.
The theory is the front brakes "are what they are" and the proportioning valve is used to "turn down" the rears so that they don't lock before the fronts. It is NOT a way to some how transfer extra pressure from the rears to the fronts, these are two completely separate circuits.
As the other have mentioned, go with the adjustable valve in the rear line only.
Adjustable is ALWAYS better. Factory valves often didn't even do any proportioning, and were very generic. A station wagon, 2dr sedan and a vert would all use the same valve, regardless of engine size. However each would require a bit different bias. Driving conditions and style can also play a role.
I have the 73 fronts, TSM rears and only the Mopar (Wilwood) adjustable valve. The fronts just have a tee. After a little tweaking, the braking performance is stellar!
Depending on your masters age, you may not have the residual valve in the master. The residual held some pressure on the drums, but later Mopar swapped to a spring in the wheel cylinder. All aftermarket wheel cylinders will have the spring, despite application, and most aftermarket master cylinders will not have the valve, but worth verifying as opposed to guessing.
Joined: Fri May 18 2007, 04:59PM
Location: Vegas
Posts: 82
FLJ: If you are sure you want to convert to rear disks, the disk specific proportioning valve will definitely be better than an adjustable drum valve. Although I doubt very seriously that brake performance will be improved over a stock disk/drum/proportioned setup.
What most don't realize is that a proportioning valve actually controls Three (sometimes Four) different parameters, known as "knee, slope, pressure, and (sometimes) delay.
Knee: is the breakaway pressure at which a valve begins to regulate line pressure.
Slope: is the amount of regulation per varying input pressure.
Pressure: is the max pressure (after the valve) that is allowed.
Delay: --- Though not found as a separate part on older proportioning valves, they do provide a little --- is the restriction of fluid flow through the valve, when pedal pressure is applied (time delay of the max pressure application to the rear wheels).
I have done two rear disk swaps over the years, and have changed them both back to stock disk/drum, as I have yet to find a rear disk swap that works as well. This is due to the lack of a 3 or 4 way adjustable proportioning valve.
Even if such a valve existed, who would/could actually take the time, or beat their car sufficiently, to test and determine what the settings should be for varying conditions (wet/dry), not to mention tire size/width, brake size differences (what is the piston diameter of the rear disks? pad area? pad material? disk diameter? etc). Even if you understand all these effects, Your car will be dangerous before you ever get everything dialed in (if ever), and you will never aproach the braking of even a modern SUV!
Adjustable valves only regulate the max pressure. The other variables are whatever the manufacturer specified, or more probably whatever they ended up at! I have only seen a couple of high dollar European adjustable valves that even list Knee and Slope values.
The car manufacturers spent substantial money to develop the brake systems in order for them to perform well over all environments.
The rear disk kit manufacturers produce what can be bolted on, with Zero testing or consideration to how they perform in the rain/heat, or if they are balanced to various front configurations. They just don't work as well as the factory disk/drum setups. Not because rear drums are better, but because 4 wheel disk cars were designed for them without the myriad of compromises that the existing kits have.
If you really want an adjustable proportioning valve, I have three MP units sitting on my shelf that I have taken off cars that others have brought to me to "fix" poor working brakes, after they "improved" them with an adjustable valve. I'd be more than happy to sell you one for cheap, but for all the reasons I listed, I would never recommend one. For that matter, I have a set of rear disks from SBBC I would sell, but only if the buyer understands that overall braking will be diminished from a stock disk/drum setup.
Save yourself a ton of money/aggravation and install a stock disk/drum/valve system. But if you don't, do not use a drum proportioning valve, adjustable or not, as you will lose a noticble percentage of your braking power, and will wear out or warp the front disks in short order.
Joined: Sun Jul 18 2010, 11:11PM
Location: DFW
Posts: 811
Funny. I converted my 65 Belvedere from 4 wheel drums and single pot master cylinder to 4 wheel discs. It works beautifully and has an excellent feel and balance in all types of weather. I installed front and rear brakes from a 2005 Mercedes S55. 14.2" front rotors with 8 piston calipers, 13" rear rotors with 4 piston calipers, a truck master cylinder with 1-1/8" bore, and a Summit adjustable valve. They've been on the car for some time and have yet to show any ill manners.
Joined: Wed Aug 11 2010, 10:15AM
Location: E WA
Posts: 1230
flj wrote ... I will be installing 1973 front discs and a Summit Racing rear disc conversion into my 1967 Polara Convertible. I am trying to figure out the proportioning valve. I believe that the stock proportioning valve will not give the rear discs enough pressure. There is a proportioning valve specifically for all disc conversions: http://www.summitracing.com/parts/RSD-PV72/ but it is not adjustable.
Howevre it appears that this valve would be installed into the rear brake line and can only reduce the pressure at that point. If I am already not getting enough pressure to the rear brakes I do not see how this helps?
What proportioning valves have any of you used with a four wheel disc conversion?
Thanks.
There is lots of good information in the replies above. I guess what I was trying to say and did not say very well is that I believe that the factory proportioning valve with a front disc rear drum setup gives the discs significantly more pressure than the drums. I will pick some number here. Lets say the discs get 1200 psi and the drums get 600 psi max pressure from the factory proportioning valve. If I go to rear discs and they need 1000 psi there is no way that they can get that if the factory proportioning valve is left in the sytstem and all that I can do with an adjustable proportioning valve added into the rear line is to drop the max psi to the rear even more. I really like disk brakes over drums and I think that it is worth the effort to put them in. On my Polara I was looking at completely replacing the rear drums, shoes, cylinders etc and figured that it would be a good time to put that money into a disc conversion. I am sure that it can be done and the result will be better more reliable brakes with much less maintenance requirements. Many A B and E body cars have had rear disc conversions. I guess that I should research more waht they did.
Joined: Tue Oct 11 2005, 01:33AM
Location: Calgary, Alberta
Posts: 5893
Seems to me that if you can lock all four wheels in a predictable fashion then your brakes are as good as your going to get. Ideally, predictable means the brakes apply uniformly with increased force until all four lock at the same time. You can't do much about left/right balance other than good maintenance. You can do a front/rear balance by decreasing the fluid flow to the rear. This works because under heavy braking, the nose dives and puts a heavier load on the front wheels. This de-loads the rear wheels and the rear brakes will lock unless the line pressure is dialled down. The fancy four factor valves attempt to keep line pressure as high as possible without lock up.
Personally I like 4 wheel disc over anything with drum brakes. First, I hate working on drum brakes, messy and way too many parts. Drums fade quicker than disc. Drums take forever to dry out in a heavy rain whereas disc are self cleaning and drying.
I am with you Bill! I love my 4 wheel disc system! I hate drums. And in my case, the rear disc swap was actually cheaper than rebuilding the drums with all new parts.
I have flogged my brakes in all manners trying to find a weak point. Nothing yet in 6 years. My Fury outperforms our 2000 Intrepid, which is also 4 wheel disc, in every possible way relating to brakes! The only issue I have has been in finding quality pads that don't leave a lot of dust on my billet wheels.
Joined: Sun Jul 18 2010, 11:11PM
Location: DFW
Posts: 811
Let me get all windy and break things down for a basic understanding.
Hydraulic brakes work through displacement of fluid. Mashing on the master cylinder squirts the fluid out to the brake cylinders. The same amount of fluid at the same pressure goes to the front and rear circuits. The proportioning valve complicates things so let's keep this simple and forget about it for a moment.
Think about what happens to the fluid when you press the brake pedal. You're running a fairly small master cylinder to push the fluid into larger brake cylinders (or pistons) so they can expand and squeeze the rotors.
Take a 6" tall glass of water. Pour it into a 5 gallon bucket. You emptied 6 inches of water but the water in the bucket sure didn't go up 6 inches. It only went up a little bit because the bucket is so much larger. Now, if you pour that glass of water into a Big Gulp cup the level will rise much higher than it did in the bucket but still not the same 6" you got out of the comparatively smaller glass.
If you think of the water in that bucket as brake fluid in a caliper you can see that the brake fluid only made a small difference. The caliper's piston will only move a small distance. The smaller caliper (the Big Gulp cup) moved a greater distance.
Let's try that with (GASP!) math. If a master cylinder has a 7/8" piston and you move it one inch you just displaced .601 cubic inches of fluid. The front brake caliper of a 1970 Chrysler (NOT Budd brakes) was 2.75". There are two front calipers so you have to remember the left and right sides share that fluid. That means your 1" of master cylinder movement just pushed the two front brake pistons a whopping FIVE HUNDREDTHS OF AN INCH. Yep! Do the math. .0507" of travel.
The TSM kit uses a Cadillac caliper with a 2.12" piston. The same 1" master cylinder movement pushes the rear pistons .085". The rears just moved 67% more than the front.
Let's change to a 1-1/8" master cylinder. The same 1" of travel displaces .994 cubic inches. The front pistons now move .0836" and the rears move .1409".
Now, go back and restrict the caliper piston's movement with, oh, a brake rotor and you've created back pressure. The pistons take up the tiny bit of slack between the pads and rotor and then stop. They can't move. The fluid doesn't compress. Your brake pedal stops moving. The bigger master cylinder is trying to shove lots of fluid into a little bitty area. You really have to strain and pop a vein in your forehead to make the car stop. The opposite happens when you go to a smaller master cylinder. It takes lots of pedal movement to get enough fluid in the pistons to take up the slack. The pedal goes to the floor and the car doesn't stop.
You're mashing on the pedal to make pressure. The pressure is the same everywhere in the brake system (again, ignoring that pesky proportioning valve). 100 psi in the brake line is 100 psi in the calipers. Remember, the fluid is trapped, just like air in your tires. You can't pull the old "it's only flat on the bottom" gag.
Pressure is measured as pounds per square inch (for us non-Canookians anyway).
The front brake caliper 2.75" piston has 5.939 square inches of surface area. The rear brake caliper 2.12" piston has 3.529 square inches of surface area.
100 psi of line pressure means 5939 pounds of clamping force against the front brakes and 3529 pounds of clamping force against the rear rotors.
Keep in mind that typical brake system pressure can approach 2000 psi. That means when the neighbor's kid jumps out in front of your car you will be applying (assuming he's not the little bastage that kicked your dog) 11,878 lbs of force against the front rotors and 7,058 lbs of force against the rear brakes.
Those pressures get divided, multiplied, added, subtracted, squished, and stretched and otherwise mangled by drag coefficients, rotor diameters, vehicle weight, road surface, the moon's gravitational pull on the earth, what color socks your neighbor had on, etc and end up being the amount of force applied to stop your car.
Remember that pesky proportioning valve? It just tossed half of all that stuff above into the trash. The back half that is. It rides along happy as a lark until you mash hard enough on the brake. Once you pass whatever point it's set at things change. After that it acts like a shock absorber and sucks up some of that line pressure. The rear line pressure still goes up in a linear progression with pedal pressure but it won't be as high as the front line pressure.
If you start monkeying around with different caliper sizes up front, out back, or both you're going to mess with the effectiveness of the brake system. The rear brakes do too much work and lock up or they don't do enough and you don't stop. This is when an adjustable valve comes in handy. You can turn the knob and change how much pressure the rear brakes see. Remember that it can only take away pressure. It can't add any.
Did you catch all that? Did you catch any of that? In short, mash the pedal and if the planets are in alignment and you did your homework, the car will stop straight and true. If not, don't worry. Take what's left of your car over to C Barge and he'll finish it right after he's done with his car in July of 2027.