The Stovebolt Site Forums The Shop Area General Truck Talk Proportioning Valve necessary for front disc brakes?

I was looking at Classic Performance Products aka;CPP, Disc brake conversion tech article this morning and the first thing that came to mind was the line pressures. Aren’t the disc brakes going to require a higher pressure to operate then the rear drum brakes? Seems like you would be locking up the rear brakes with this set up. Maybe I just missed it when I skimmed over the instructions but I don’t think they didn’t make any mention of the need for a proportioning valve if you install the disc brake conversion kit.

No, I’m not throwing in the towel; I’m still the same old purist I always was. It’s just that I’m always interested in what others are doing and might be asked by someone to help out someday.
Denny Graham
Sandwich, IL

You may not need a proportioning valve but you will need residual valves at the very least. I think the pressures are 10 psi for the rear and 2 psi for the front. Check with Classic again and they can tell you for sure.

Not exactly sure which tech article you were looking at or which brake conversion components you had in mind, but most of CPP's master cylinder and master cylinder/booster setups come with a proportioning valve already installed inline.

If you take a look a some of the pictures they include with the parts, the proportioning valve is the brass block that is about 1"x2"x4" that is normally mounted to a plate just below and/or in front of the master cylinder. The brake lines run from the discharge ports on the master cylinder to the proportioning valve and then out to the brakes.

Hope this helps.

There are a number of suppliers out there selling these kits and many are set up correctly and some not so good. I don't use CPP's stuff but they have an impressive catalogue. The proportioning valve is an important part of a disc conversion but they can be the adjustable type installed in the rear brake line. The brass combination/proportioning units are my choice. If the supplier sets up the master cylinder correctly it will have the residual valves built in which eliminates the need for additional plumbing for external valves.
Fred

This is off ECI's site. http://ecihotrodbrakes.com/brake_facts.html

Hold-Off / Metering Valves are used in the front (disc) system of a disc/drum brake system. They provide a "hold off" function to allow the rear (drum) brakes to actuate first. This function is very important in making the system function in the correct sequence in a rear wheel drive car. The rear brakes are always actuated first. This function is built into most factory type disc/drum combination valves. Make sure you have a metering/hold-off valve in the system either as a stand alone valve or as part of a factory valve.

Proportioning Valves go in the rear brake system (disc or drum) and provide for control of the rate of pressure rise to the rear brakes -- just the rate at which it builds up. Sooner or later the rear brakes see full master cylinder discharge pressure. The purpose of this rate of pressure rise control is to compensate for the reduction of weight on the rear wheels due to forward weight transfer during braking. In short, it eases the application of the rear brakes to help prevent rear wheel lockup. Factory combination valves have these built in -- make sure you know what you're getting, too little rate of rise is as bad as too much. You may not need one of these valves depending on the compatibility of your vehicle's brake system components.


Adjustable Proportioning Valves allow for fine tuning of the rate of pressure rise to the rear brakes if you have a lock up problem. If you have a lockup problem, experiment with the setting of the valve to eliminate lock up for all but all out panic stops. Good luck!

Residual Pressure Valves are used in a both front and rear brake system as follows:

2 PSI Valves - These valves are used in a disc brake system only and are required when the master cylinder is at, or below, the height of the calipers. It's purpose is to act as an anti-siphon valve preventing the brake fluid from siphoning back into the master cylinder when the brake pedal is released. Even if the master cylinder is even or slightly above the calipers, put one in anyway. If you don't and you park on a hill, fluid will siphon! These valves are cheap insurance - put them in!
NOTE: You will know if you need one of these valves if you had to pump the pedal twice to get a good pedal. See illustration for more.
10 PSI Valves - These valves are used in a drum brake system to prevent air from being ingested into the hydraulic system when you release the brake pedal. Typical wheel cylinder seals only seal when there is pressure behind them. Rapid release of the brake pedal creates a vacuum in the system which causes the seals to relax and air is ingested into the wheel cylinders. Maintaining 10 PSI in the system at all times prevents this. Some disc/drum master cylinders have 10 PSI residual pressure valves installed internally, some don't. If you're not sure, call us and we can tell you how to check. Also, some new style wheel cylinders have cup expanders which negate the need for the residual pressure valve. Either way, if you are not sure whether you have one or not, put one in. They are not cumulative and it won't hurt anything if you have two. Don't worry about brake drag, it takes roughly 75 PSI to overcome the return springs.

Yep, that's exactly why I'm perfectly happy with me old Huck brakes, nice and simple. Thanks for the link Lonnie.
Denny G

Did someone say CPP? :hammering:

Yes, I did in the first line Woodsman! Don’t tell me that your thinking of making a Silverado 1500 out of your 3100???
Denny G

Go wash your mouth out with soap and go to your room... your father will deal with you when he gets home.

I don't have a proportioming valve in mine, and they work great. I have Mustang II IFS with MustangII master cylinder set up. It'll stop on a dime.

Denny, you are right. It looks confusing. The basics are if you change to disks in front and the MC is in the stock position, the residual valves, 2# in front and 10# in rear are needed and an adjustable proportioning valve for the rear brakes.

If you change to disks front and rear, 2# residual valves are needed at both ends.

And ya never lock up the rear's Pablo??? That was my concern. mind you now I'm not doing this to mine and have no intention changing mine. I really am perfectly happy with the Huck brakes after a total, I mean total rebuild of every part on the truck including the emergency that also works great. I help out other guys once in a while and just was adding to my database.
Thanks for the discussion guys.
Denny Graham
Sandwich, IL

Lonnie,
I feel the need to agree with you, you got it covered when you point out that the frt. disc need to hold off till the rear drums have time to move and get applied before the disc do up front.
Otherwise you'd be braking with the frt disc first.
Dan

Denny,
Nope the rears never lock. Got the idea from a friend who did the same thing in his '53 panel. Don't know if the Camaro rear end has anything to do with it or not, but I have absolutely no problem stopping and it stops straight.

Since the pressure in a single line system is the same at all cylinders the pressure build up at all four cylinders will not peak or build until there is resistance at all four cylinders. Meaning that the pressure at the front disc will not build till the rear shoes hit the drum and therefore the braking will occur at the same time for all four wheels. Now how much braking will be applied is the question.
The differences that will present themselves with a "hybrid" system, if I can call it that, is in the cylinder piston area, the friction area applied to the rotating member, the mechanical design of the brake system components and the factor of weight transfer, i.e., most of it being transferred to the front wheels during braking.
Disk brakes require a lot more pressure to stop the wheel than does a drum brake. This you would know if you have ever driven a disc brake vehicle without power assist. Of course their advantage is that they don’t suffer from the brake fade that a drum drum brake does. It's for these reasons that I believe with enough pressure applied to the front (disc) brake that the rear (drums) would have a tendency to lock up especially during heavy (panic) braking or on slippery surfaces. This reasoning is what lead me to question the installation documented on the CPP site. I’m sure if one called them on this that they would agree that a means of adjusting the proportion of pressure to the front and rear is highly recommended.
Simply pulling the front drum-brakes and replacing them with discs with no other plumbing changes just doesn't sit right with me.
Lots of opinions surfacing on this one guys. Seems like ya gotta apply some common sense along with that hydraulic pressure.

Denny Graham
Sandwich, IL

Denny,

Here is a brake article that I believe addresses your question. It is on valves in brake lines and includes several graphs comparing torque pressure and line pressure in several combinations of drum and disc setups.

You may find it helpful.

http://www.thebrakeman.com/valvetechi

I use a Mustang II dual master cylinder without power assist, disc front, drum rear. The line going to the front is 3/16" (if I remember right), and 1/4" to the rear. Been running this for 10 years of almost daily driving and the brakes have worked great in all sorts of stopping circumstances.

Hey Randy, thanks for the link, looks pretty interesting. A lot to read, I printed a copy for the throne room, I'll look at it later.
Well now your not simply replacing the front drum brakes with discs like was refered to in the article Pablo. Now your into a dual master cylinder and that's quite a bit different than the standard '49 master cylinder. Might account for the performance of your system.
Denny Graham
Sandwich, IL

Denny,

I think your questions about the CPP article come about because they assume you will be running a modified system with dual master cylinder chambers and you are theorizing about the problems that would come about from using the single chamber stock unit to actuate both the front discs and the rear drums.

I bet you gave your teachers fits with your questions when you were in school.

Apparently CPP makes that assumption; they sure don’t make any specific reference to it in their tech tips. I’ve got a buddy who has a ’50, 3100 and he intends to do a lot of upgrading to it. Maybe I can help him out now.
I think I’ve got a pretty good picture in my mind, at least enough to say my mind is made up as to what direction I would go in, if ‘n I ever reverted back to hot rodding that is. And I think I could get a passing grade on this subject now so it’s about time to move on to the next flower. Thanks for the discussion all.
Denny Graham
Sandwich, IL