69 Proportioning Valve Location - Team Camaro Tech
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post #1 of 26 (permalink) Old Jul 30th, 01, 02:01 AM Thread Starter
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Gary
 
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The "Fake RS/SS" post was getting into Prop valve questions, so thought I'd start fresh.
My car started out plain Jane, so the guy I bought it from had converted to front discs. The prop valve was on the cowl. I got another valve off a junker SS, and it also was on cowl. Everyone talks about looking for the prop valve on the frame, under driver door.
Obviously, both setups were used, but can anyone define when one setup was used over another?
Can't be Convt vs Coupe, I've seen coupes both ways.
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post #2 of 26 (permalink) Old Jul 30th, 01, 03:56 AM
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The valve mounted under the drivers seat on the frame rail is not truly a proportioning valve, rather its a surge suppressor or pressure balancing valve. It was used on cars equipped with power front disc's and is actually part of the RPO L48 option.

It started out in 67 on big block cars and V8 cars equipped with A/C due to the front end weight increase as a resolution to the rear brakes tending to lockup. It's purpose is to reduce the pressure spike in the system when the brakes are applied quickly. The valve is not widely used on 68 cars but made a comeback in 69.

The valve was used on SS's and Z28's. However, like everything else with Camaros there are periods of time where the valve disappears from use, and then returns and also Los Angeles cars seem to not use them as much as Norwood Cars.

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post #3 of 26 (permalink) Old Jul 30th, 01, 08:38 AM
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Unreal,

The valve mounted on the cowl beside the master cylinder is called a metering valve in the Chevy shop manual. It is part of the front disc circuit and allows the rear drums to receive pressure before the front disc. These valves only came on disc brake cars. Some people mistakingly refer to it as a proportioning valve.

If you find a metering valve, pick it up. They are getting very hard to find (i.e., expensive). Anyone that wants to do a stock looking front disc conversion MUST have one of these little doodads.
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post #4 of 26 (permalink) Old Jul 30th, 01, 11:10 AM Thread Starter
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I pulled a "metering valve" and splitter block off a junker, in Florida (since gone to the crusher) I never looked under the car to see if it also had a valve there. However, I did look at one of the cars at Ricks, one time. It had no valve at the booster, but did have one under the car. I thought it was one or the other.
Also, Rick's catalog and D&R's catalog show the valve I have, and call it the "proportioning valve" (same as PN 3905525 in the Assembly Manual)
D&R also shows a combination metering block and proprotioning valve, which looks similar to what I remember the valve under the driver door to look like. I'm still a bit confused.
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post #5 of 26 (permalink) Old Jul 30th, 01, 12:15 PM
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I think Rick's car is a JL8, hence no metering valve.

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post #6 of 26 (permalink) Old Jul 30th, 01, 12:35 PM
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How timely! I had a guy look under my car last night at a cruise and ask me about the "valve" on the frame, under the front drivers side. He was looking at he floors and noticed the valve. I thought it was a proportioning valve, He has a 69 Firebird and said he does not have one. I have been asked that question by a few people over the years and I had no answer. The car is an original 30000 mile Z/28 so I knew it was stock. But rare?

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post #7 of 26 (permalink) Old Jul 30th, 01, 01:57 PM
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I just bought a rebuilt metering valve from Ricks. I did notice that they incorrectly called it a proportioning valve in their catalog.

The metering valve is simply a needle held in a seat by a spring. The metering valve restricts fluid flow and, therefore, pressure from the master cylinder to the front disc system until enough pressure builds to overcome the spring inside the valve. At that point, the front disc system gets full flow and pressure. So, you could say that a metering valve acts as a proportioning valve until the preset pressure is exceeded and the valve opens. I understand Camaro metering valves require 40-50 lbs of pressure to open.

The proportioning valve is a pressure regulator, similar to a fuel pressure regulator. It restricts fluid volume and pressure going to the rear brakes at all times.

A combination valve has both a metering valve and a proportioning valve.
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post #8 of 26 (permalink) Old Jul 30th, 01, 02:57 PM
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My 68 has 4-wheel power drums and it has the proportioning valve. I thought it was on all air-conditioned models and most SS's in 68. My car is an SS with factory air.

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post #9 of 26 (permalink) Old Jul 30th, 01, 03:16 PM
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I've got a 68 junker that had AC and auto trans, I don't know the engine, but the chassis number is 12437 which indicates a V8. It has drum brakes, and no valve on the frame.
My Helm factory manual for 67, in the drum brake section, calls the valve on the frame a "Pressure Regulator Valve.
Says it was for use in AC cars - "controls the hydraulic pressure to the rear brakes resulting in the correct pressure balance betweeen the front and rear hydraulic systems".
The Disc Brake Section has different text and says the valve is located just under the master cyl, and "in the FRONT brake line".
They even show an illustration of the master cyl and valve, but it is what we call the metering valve.
-"models with disc brakes have a pressure regulator valve in the FRONT brake line just below the master cylinder. The valve controls the hydraulic pressure to the front or (rear) brakes as applicable" (they mentioned the corvette having a valve in the rear line above this sentence) "resulting in the correct pressure balance between the front and rear hydraulic systems. This valve guards against premature lock up of front or rear wheels when brakes are applied."
You can read a clip of the drum brake section of the manual manual here: http://ourworld.compuserve.com/homep...rop_manual.jpg
I've got a friend with a 69 Z/28 that had the valve and no AC of course.
I'm not disputing anything said about this valve at this point, just trying to figue out what is going on, what it does, so I can put accurate info on my brake page.
Front discs require much more line pressure to operate because they are not self engergizing. Something has to reduce the pressure to the rear drum brakes to keep them from locking first. some proportioning is done by using a smaller wheel cyl in the rear but disc line pressures are almost twice what drum pressures are, so if disc/drums work without a prop valve, the rear drums would be way too low for a drum/drum system.
It obviously isn't the metering valve attached to the master cyl, because it is in the front brake line, not the rear line.
If someone who has this valve on their car would unscrew the plug and remove the valve inside, replace the plug, bleed and test drive the car and tell us what difference it made we might learn more about it's function.
David

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[This message has been edited by davidpozzi (edited 07-30-2001).]
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post #10 of 26 (permalink) Old Jul 30th, 01, 04:26 PM
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Heres the quote from the 69 Chassis Service Manual.

" On Camaro models equipped with 8-7/8" ring gear, the rear brake line is routed through a pressure regulator valve mounted on the left frame rail. The valve controls the hydraulic pressure to the rear brakes resulting in the correct pressure balance between the front and rear hydraulic systems."


Thats why SS's and Z28's are the only cars (unless you ordered power front disc's and a 12 bolt rear axle together) that get the valve on the frame. A power disc brake car (RPO J50/52) will have the distribution block directly below the master cylinder and the metering/proportioning valve below that. but it will not have the regulator valve on the frame rail.

What made the 12 bolt rear axle require the extra valve is the mystery.


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Mark Canning
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post #11 of 26 (permalink) Old Jul 30th, 01, 06:29 PM
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I was speaking with a friend of mine who works for Wilwood and we were getting together all of the components for my disc conversion. I will be running a front disc/rear drum setup. He says that on a disc/drum setup you will need a Proportioning valve to adjust pressure from front to rear. You also need a residual pressure valve to keep a bit of pressure on the rear drums. Just thought I would throw in the info I have.

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post #12 of 26 (permalink) Old Jul 30th, 01, 07:45 PM
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My 69 Z is missing this valve on the frame rail. Noticed it was missing a long time ago and so I picked one up. Just have not installed it yet. Every once in a while my rear brakes will lock-up. It has only happened when I'm just cruising slow through a neigborhood and apply normal pressure to my brakes. But most of the time they work just fine. It's usually a suprise to me when it happens. It never happens when I apply the brakes at hwy speeds. I'll add that little valve to my car one of these days... I can tell there used to be one there before.

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post #13 of 26 (permalink) Old Jul 31st, 01, 02:40 AM
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I bought my 69 Z/28 in 1982 from the original owner, unmolested and It does not have the valve on the subframe, brakes work fine, My 67 covertible, V8 with a/c has one, and it had a 10 bolt rear, I remember reading in an old Camaro Corral years ago that big block cars and those with the additional weight of the a/c equipment had them installed, but not all the time, something about delaying the application of the rear brakes to allow a dive of the front of the car for weight transfer. I will try and find the article in my old mags.
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post #14 of 26 (permalink) Old Jul 31st, 01, 08:58 AM
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Although I don't have any GM documents to verify it, I talked to a retired GM engineer at a local show last weekend who had worked in the Camaro Chassis Group, and he indicated that the valve on the frame limits maximum pressure to the rear drums (due to the higher pressure required for the discs in the front system) so that the rear drums won't lock up before the front discs do. He said that Chevrolet design guidelines called for front lockup first under ALL conditions as a safety consideration - if the rears lock first under panic braking, the rear can come around and cause a spin; the fronts locking first just lets the car plow straight ahead, with the rear tracking straight behind.

I asked him about the seemingly-random application of the valve between options and the two plants, and he said there were lots of running changes in the application over the years for various reasons, but it was a long time ago and he couldn't answer it without reconstructing all the paperwork and ECR's that describe the reason for each change, all of which is long gone.

Sounds logical to me, as "fronts locking first" has ALWAYS been the OEM industry design guideline.

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post #15 of 26 (permalink) Old Jul 31st, 01, 12:55 PM
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reversing the brakelines at the master on a disc drum setup would make the larger piston work the front breaks. <BLOCKQUOTE>quote:</font><HR>Originally posted by JohnZ:
Although I don't have any GM documents to verify it, I talked to a retired GM engineer at a local show last weekend who had worked in the Camaro Chassis Group, and he indicated that the valve on the frame limits maximum pressure to the rear drums (due to the higher pressure required for the discs in the front system) so that the rear drums won't lock up before the front discs do. He said that Chevrolet design guidelines called for front lockup first under ALL conditions as a safety consideration - if the rears lock first under panic braking, the rear can come around and cause a spin; the fronts locking first just lets the car plow straight ahead, with the rear tracking straight behind.

I asked him about the seemingly-random application of the valve between options and the two plants, and he said there were lots of running changes in the application over the years for various reasons, but it was a long time ago and he couldn't answer it without reconstructing all the paperwork and ECR's that describe the reason for each change, all of which is long gone.

Sounds logical to me, as "fronts locking first" has ALWAYS been the OEM industry design guideline.

<HR></BLOCKQUOTE>

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