Drag car brake/proportioning question

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demon322

demon322

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The race car I bought recently does not have a proportioning valve for the brakes. Is this common? Does it need one? I haven’t had a chance to weigh it yet but it’s fairly light. 71 Demon. Fiberglass front cap, hood, deck, and doors. Lexan windows. 4 wheel disc.

it does have line lock installed and this fitting in the line going to the rear brakes which I am not sure what is.

Thanks all.

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Someone may correct me. 4 wheel disc brakes or 4 wheel drum brakes does not require a proportioning valve. A proportioning valve is used when one has disc brakes on the front and drums on the rear. As discs will brake harder than the rear drums will which will be catrostophic.
 
You should at least have an adjustable prop valve in the mix somewhere. The valve you show is probably a residual pressure valve, which maintains pressure in the lines for the rear brakes. That's typically only a drum brake deal, they do better with a residual brake valve because they stay "staged" so they act faster. With disks a residual valve isn't necessary unless the master cylinder is mounted below the level of the calipers/lines, and should only be a 2 psi valve if that. With too much residual pressure the disks will drag.

The adjustable prop valve is nice because there wasn't any factory 4 wheel disk system for these cars. The 4 wheel drum cars didn't have a proportioning valve, just a distribution block. But the brakes were designed/sized to work without one, so the front rear bias didn't require one to be balanced. With an aftermarket disk set up the you may not be using disks and calipers off the same car or application, so there's no guarantee that the front/rear bias will be correct for your car. An adjustable valve would let you reduce pressure to the rears so you don't end up *** end first with a hard brake application.
 
Enjoying it again said:
Someone may correct me. 4 wheel disc brakes or 4 wheel drum brakes does not require a proportioning valve. A proportioning valve is used when one has disc brakes on the front and drums on the rear. As discs will brake harder than the rear drums will which will be catrostophic.
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Uh, no. You WANT the front to brake harder, or you WILL spin out. That's why discs were introduced on the front of the car, to keep the bias correct. The fronts do something like 75% or more of your braking. A prop valve keeps the rear from locking up first.
 
I ran for several years without a prop valve. I factory disc's up front and wilwoods's in the rear. U always had a bad shimmy from the front under hard braking running 1/4 mile. Someone said that I may have to much braking in the front and it's on the verge of locking them up. Dr. Diff said this may may be a problem and would have to put a adjustable one on the front brakes only, which I already plumed in for this year.

From what the Dodge manual said about my old truck is that the factory prop valve sends fluid to the rear first. Once it reaches xx internal pressure it then sends it to the front. I would imagine that's how they all work.
 
I know Cass at Doctor Diff recommends a distribution block for all disc, or all drum set up, but I think that is really for street set ups. That said, when I had my drag car, I ran an adjustable prop valve on my all disc car. I do remember having to actually put more braking to the rear then normal since the front skinny tires did not have a lot of traction for stopping. Every car is different, but having the availability for adjustment is nice if you need it.
 
73smallblock said:
I ran for several years without a prop valve. I factory disc's up front and wilwoods's in the rear. U always had a bad shimmy from the front under hard braking running 1/4 mile. Someone said that I may have to much braking in the front and it's on the verge of locking them up. Dr. Diff said this may may be a problem and would have to put a adjustable one on the front brakes only, which I already plumed in for this year.

From what the Dodge manual said about my old truck is that the factory prop valve sends fluid to the rear first. Once it reaches xx internal pressure it then sends it to the front. I would imagine that's how they all work.
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No, that’s not how they all work. All the factory stuff for these cars is intended for use with a disk/drum set up. How it’s balanced and proportioned was intended to keep the disks and drums, which take different amounts of time to apply, working in unison.

If you’ve gone to aftermarket brakes, 4 wheel disks etc then adding an adjustable valve is the only way you’re going to get the front and rear balance set properly for your car. Maybe running with the factory distribution block or prop valve will get you close enough for most braking situations, or maybe you’ll get lucky and it’ll be right. But that’s unlikely, especially when you start mixing and matching aftermarket set ups.
 
demon322 said:
The race car I bought recently does not have a proportioning valve for the brakes. Is this common? Does it need one? I haven’t had a chance to weigh it yet but it’s fairly light. 71 Demon. Fiberglass front cap, hood, deck, and doors. Lexan windows. 4 wheel disc.

it does have line lock installed and this fitting in the line going to the rear brakes which I am not sure what is.

Thanks all.

View attachment 1715508580
Click to expand...


That looks like a residual pressure valve which limits the pressure to the rear. Look close at it as it should have a PSI value.

Residual Pressure Valves | TBM Brakes
 
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demon322 said:
The race car I bought recently does not have a proportioning valve for the brakes. Is this common? Does it need one? I haven’t had a chance to weigh it yet but it’s fairly light. 71 Demon. Fiberglass front cap, hood, deck, and doors. Lexan windows. 4 wheel disc.

it does have line lock installed and this fitting in the line going to the rear brakes which I am not sure what is.

Thanks all.

View attachment 1715508580
Click to expand...
Not to mention those sections of brake line look rough.
 
The valve limits the pressure to the rear but is infinite. Factory type Prop valves generally limit pressure for a short time allowing the fronts to grab harder on brake pressure. You will never get full rear braking using these but generally safe for drag racing if the PSI value is good for the combo!
 
72bluNblu said:
With disks a residual valve isn't necessary unless the master cylinder is mounted below the level of the calipers/lines, and should only be a 2 psi valve if that. With too much residual pressure the disks will drag.
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The master cylinder is mounted in the floorboard so it is low, close to in line with the calipers and lines.
 
A56 said:
Not to mention those sections of brake line look rough.
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I agree! My plan was to replace all lines before I hit the track (hopefully) this summer so that’s why I wanted to ask. I knew it would be a great time to add a prop valve while I’m at it.
 
Thanks for all the thoughtful replies guys!

So if I go with a proportioning valve can I eliminate that residual valve or should I run both? Is this type ok? That way it could mount under the car with the MC and the adjustment could be in the cab.

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AAAF1F9A-CDD9-4004-AF1C-FFF41F5ACBC6.jpeg
 
demon322 said:
The race car I bought recently does not have a proportioning valve for the brakes. Is this common? Does it need one? I haven’t had a chance to weigh it yet but it’s fairly light. 71 Demon. Fiberglass front cap, hood, deck, and doors. Lexan windows. 4 wheel disc.

it does have line lock installed and this fitting in the line going to the rear brakes which I am not sure what is.

Thanks all.

View attachment 1715508580
Click to expand...

It is complicated! That is why there are rooms in Detroit full of engineers who do nothing more than design brake systems. With four wheel disc brakes you do not necessarily need a prop valve. A prop valve reduces the brake pressure to rear drum brakes. A disc/drum setup needs a prop valve since disc brakes are linear and drum brake regenerate. With disc brakes in the rear you have a linear system all the way around so you do not need a prop valve.

BUT, that is only true if the brake system is properly designed. Seeing the picture you posted tells me that your system was built by a hack not a chassis shop. Which means that it most likely is not properly designed so you'll have to do some testing before you'll know what to do. The proper balance of braking between the front and the rear depends on weight distribution, how high the center of gravity is and the available traction at each end of the car. A drag car with skinny tires up front an big slicks in the back might want more brake force on the back than a passenger car. Nobody on this forum can help you since we have no idea what your weight distribution is or how high the CG is or what the available traction is. Any answers you get are just guesses since we don't have any facts.

Get the car running and driving and then see how the brakes feel. If one end of the car is doing most of the braking then you'll have to make an adjustment. The best way to adjust it is to put larger rotors on the end that you want to increase braking force. Or you can use calipers with larger pistons in them. Typically you want a 66/33 split of brake force front to rear but that ratio is dependent on the stuff I mentioned before.

Take care, try it out slowly and see how it goes. There are a lot of different rotors and calipers available so you'll most likely end up just mixing and matching to find the correct balance.
 
demon322 said:
Thanks for all the thoughtful replies guys!

So if I go with a proportioning valve can I eliminate that residual valve or should I run both? Is this type ok? That way it could mount under the car with the MC and the adjustment could be in the cab.

View attachment 1715508864

View attachment 1715508865
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Why in the hell is the master cylinder under there? There's no way I'd leave it there. With a rubber flap around the hole in the floor? Geez.

If you did leave it there, yes, you might need to retain a 2psi residual valve because the master is going to be lower than the calipers are. Residual valves are commonly either 2 psi for disks or 10 psi for drums. But yeah, I don't like the looks of that set up at all.

The adjustable proportioning valve can be run on the rear line under the floor with the adjuster in the cabin if you really need it to be. Unless you're constantly changing your weight balance, suspension settings or tire compounds that's not really necessary, you can set it and then leave it. If you are constantly changing those things then remember to be really dialed in you'd need to test and adjust the brakes every time you changed any of that stuff.

As Andy said, if you have a brake system designed for your car then you may not need one. But aftermarket brakes are generally adapted for use on different platforms, so, just because you have a kit that fits your car doesn't mean it was designed for it. So the adjuster allows you to dial in that balance if you need to. It can only reduce pressure to the rear brakes though. And if it turns out that you don't need it because by some stroke of luck your brakes are perfectly matched to your car set up, you can just leave it wide open.
 
72bluNblu said:
No, that’s not how they all work. All the factory stuff for these cars is intended for use with a disk/drum set up. How it’s balanced and proportioned was intended to keep the disks and drums, which take different amounts of time to apply, working in unison.

If you’ve gone to aftermarket brakes, 4 wheel disks etc then adding an adjustable valve is the only way you’re going to get the front and rear balance set properly for your car. Maybe running with the factory distribution block or prop valve will get you close enough for most braking situations, or maybe you’ll get lucky and it’ll be right. But that’s unlikely, especially when you start mixing and matching aftermarket set ups.
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Het blu, wouldnt bigger rear tires cause more braking in the rear w/ just a factory distribution block ?
I`ll try to keep the following short--------------unrelated, but I bought a new 1968 , 383 form S fastback in 1968 , it had factory front discs and drums in the rear/were the only power option on the car. I was on Tinker A F B and came up on a very good looking young lady wearing the shortest mini skirt I ever saw in public at that time, her *** was beautiful !
I looked back just in time to stomp my brakes , the guy in front of me was stopped dead in the road looking at her. The front didnt lock up , but the back did and the car turned almost completley sideways in the rear , stopped about a papers width from hitting him, which would have been my fault, not to mention wrecking a brand new car . ''end of story''.
 
demon322 said:
Thanks for all the thoughtful replies guys!

So if I go with a proportioning valve can I eliminate that residual valve or should I run both? Is this type ok? That way it could mount under the car with the MC and the adjustment could be in the cab.

View attachment 1715508864

View attachment 1715508865
Click to expand...

I put one of those on a hot 406 vega, and one one a 406 sbc GMC , it about did nothing to the rear brakes w/ big tall tires on it --------------??s
 
The residual pressure valves are there because the master cylinder is frame mounted. This was a common problem with street rods where the master cylinder was low mounted. IIRC a 2 lb valve was used for the rear brakes & 1/2 lb was used on the front along with a proportioning valve. It's been a while since I've worked on that type of system. I had a street rod with Mustang II manual front disc brakes & drum rear brakes with just a proportioning valve that "seemed" to work great under normal braking. The master cylinder did not have the residual valves in it. The first time I had to get on the brakes hard, the pedal went to the floor. Two quick pumps & they were back up. I did some research & talked to some other street rod owners/builders. I put the residual valves on my car & all was good with the world.
 
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I built the brake system on my Duster and so far it seems to work but I haven't done any high speed full brake tests. I used 13 inch rotors up front with Viper calipers and a Ford Motorsports Explorer disc brake kit in the rear. I used a Baer billet master cylinder with a built in prop valve. At the moment I have the valve fully unscrewed so I have full pressure to the rear. I'm not sure that I need a prop valve in my Duster but the master cylinder came with one so I plumbed it in. When you custom build cars you have to be willing to try different things to find out what works since there isn't a blueprint. It isn't like restoring a factory car where someone has already done the design work. Store bought kits might not work since your car might have a different weight or a different weight distribution from what the kit was designed for.
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I built my system with Wilwood 4 wheel manual disc with a distribution valve & no proportioning valve. The car weighs 2500 lbs with me in it, 2 batteries & 5 gallons of fuel. Weight bias at 50/50, brakes slow the car like a champ. There is an 1/8th mile track close to me with a very short shutdown area, I can make the middle turn out with no problems.
 
AndyF said:
your system was built by a hack not a chassis shop. Which means that it most likely is not properly designed so you'll have to do some testing before you'll know what to do.
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I appreciate your reply.

I agree that it is complicated. I don’t know if I’d say the car was built by a ‘hack’ but I don’t blame you for coming to that conclusion based on that picture. It is an old build for sure and from what I can gather the car was built in sections. Back half first, tube front end last. I agree that I will need to do some testing and experimenting once it’s running. It was raced in this condition but that’s no excuse not to do it better this time around so I do thank you for helping me plan my attack here
 
72bluNblu said:
Why in the hell is the master cylinder under there? There's no way I'd leave it there. With a rubber flap around the hole in the floor? Geez.

If you did leave it there, yes, you might need to retain a 2psi residual valve because the master is going to be lower than the calipers are. Residual valves are commonly either 2 psi for disks or 10 psi for drums. But yeah, I don't like the looks of that set up at all.
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I wondered that myself as this is the only car I’ve had with this type of setup. I assume it was put there because the firewall was cut out and moved back and the brake pedal is floor mounted instead of hanging under the dash. So the rod comes out of the front of the MC and attaches to the brake pedal. I plan on leaving that all as is. So I can keep the residual valve and plumb in a proportioning valve when I replace the lines in case it is needed. The front has wilwood hubs and discs and a mustang II front end. I will check the rears when I get back to the shop but I think they are aftermarket pieces as well.
 
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