[GTO]

I want to install an adjustable prop. vlv. on my 69 GTO. There is a small round vlv. like unit inline from my master cyl. front disc reservoir and the distribution block on the frame. Is this the original prop. vlv.? This used to be a power disc brake car but last year I installed a 68 standard disc master cylinder. Was there anything else I should have changed? The brakes feel OK but I would like some adjustment to maximize my system. It's a pro street car with the big & little tire combination. Thanks...Eric V.

[GTO]

I want to install an adjustable prop. vlv. on my 69 GTO. There is a small round vlv. like unit inline from my master cyl. front disc reservoir and the distribution block on the frame. Is this the original prop. vlv.? This used to be a power disc brake car but last year I installed a 68 standard disc master cylinder. Was there anything else I should have changed? The brakes feel OK but I would like some adjustment to maximize my system. It's a pro street car with the big & little tire combination. Thanks...Eric V.

[JC455]

Eric,
That sounds like your factory prop valve.
The adjustable unit should really help out with the rear brakes. A little tip, if you buy through Summit, You can get their version (looks just like Wilwood's- just with their name on it) for a lot cheaper.
Hope this helps-


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John

[jnyeager]

Eric, the device you described is not the factory proportioning valve. It is the metering valve. This device is there to ensure that the rear brakes receive about 75 psi of pressure prior to the front brakes receiving their pressure. This is because it takes drum linings longer to contact the drum than it takes disc pistons to contact the rotor (because they have to overcome the return springs). The proportioning valve is the 'junction block' on the frame that you described.

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[JC455]

JNYEAGER,
I'm not disputing you, but isn't the object of the proportioning valve to meter fluid to the rear brake in a different proportion to the front? I thought that the junction block on the frame was simply a distribution junction, and that the "metering valve" above it is was the proportioning valve. As I understand it, there are residual check valves located in the master cylinder that ensure drum brakes retain a certain amount of pressure so they can overcome the return springs.
I know that the later (70's) cars had a combination of proportioning and distribution block that was attached to the power booster, and the lines went straight out from there, thus eliminating that valve.
If I'm wrong, I would appreciate your input.
Thanks!


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John


[This message has been edited by JC455 (edited 01-06-2000).]

[This message has been edited by JC455 (edited 01-06-2000).]

[jnyeager]

My information comes from my 1970 Pontiac service manual. I believe the setup is the same for your application. Here is the truth as I understand it...

There is a residual pressure retained in the drum brake systems, but this is not to overcome the return springs, this is to keep the return springs from pulling air into the system through the wheel cylinders.

The proportioning valve gives different pressure to front vs. rear for obvious reasons.

The metering valve is for the purpose I described, to ensure that the front to rear brake application happens at the correct time by compensating for the greater time it takes drums to apply over discs. Notice this metering valve would not be used for a 4-wheel drum setup.

The first device, located in close proximity to the master cylinder is the metering valve. The proportioning valve is mounted on the frame rail and also contains the switch for the brake warning light.

If you have any more specific questions, I can directly reference my service manual for you. This assumes your application is the same as 1970 in this matter which I believe it is.

Please e-mail me if you want to talk, I'll be glad to help in any way I can. jnyeager@iol-12.net

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[JC455]

Boy do I have egg on my face! First off my apologies (sp?) to JNYEAGER- it seems he was correct on the metering valve's function. I was unaware of this because (as I stated...) 70's technology put the proportioning valve/distribution block/safety light/METERING VALVE into the combination valve. I was searching for some info on a different type of disc brake conversion and found the info on (and a cut-away of) the combination valve. As JNYEAGER stated, drum brakes don't require this because of obvious reasons. It's still a good idea to use the adj. prop valve. [img]/infopop/emoticons/icon_smile.gif[/img]



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John