proportioning valve AND distribution block ??? - Chevelle Tech
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post #1 of 7 (permalink) Old Jun 4th, 18, 2:26 PM Thread Starter
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Phil
 
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proportioning valve AND distribution block ???

okay... putting power discs on the front of a 1969 factory manual drum car. I'm using the "'71/'72" style rectangular combination valve up by the master cylinder. Can I leave the original "distribution-block" in place down on the frame? Does having 2 blocks now functioning as "distribution blocks" cause any issues ? Should I plug the brake warning wire into the new combination block on top/ or into the original block down on the frame? or, doesn't that matter?

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post #2 of 7 (permalink) Old Jun 4th, 18, 7:26 PM
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Denny
 
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Re: proportioning valve AND distribution block ???

No, you shouldn't use those two together. Especially if you have a prop valve in the combination valve (and you should have one).

With a prop valve, when you will apply the brakes hard, the rear line pressure will be lower than the front one. This will trigger the pressure differential valve in the original distribution block, which may restrict even more the rear flow (and trigger the brake warning light if you are connected to the distribution block).

Remove the original distribution block and connect new custom brake lines from the combination valve to the original brake lines (with double flare brake line unions, see below), normally connected to the distribution block outlets. For the front ones, it might be easier - and cleaner - to just ran new lines directly to the wheels.


Left: Double flare union (high pressure), YES!
Right: Copper tubing union (low pressure), NO!


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post #3 of 7 (permalink) Old Jun 4th, 18, 9:55 PM
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Bob
 
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Re: proportioning valve AND distribution block ???

Something else to consider. Not sure what you are working on? On 1970s with P.D. Brakes............ Left rear frame rail, You will find a RESTRICTOR block ( it looks like a brass union to attach brake lines). This block restricts flow to the rear so the rear brakes DO NOT lock up and turn the car around.
I repete, This is 1970 power D. Brks...... NOT sure of plumbing for other years. Touch base or ask question about your year and brake type.
Bob


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post #4 of 7 (permalink) Old Jun 5th, 18, 7:01 AM
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Denny
 
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Re: proportioning valve AND distribution block ???

Quote:
Originally Posted by Robinls5 View Post
Left rear frame rail, You will find a RESTRICTOR block ( it looks like a brass union to attach brake lines).
That is a proportioning valve. There was none before 1970. Starting in '71, it was incorporated into the combination valve. So 1970 is the only year you will find this valve.


Having 2 prop valves in series would be a big no-no, as well. It can lead to unpredictable results.


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post #5 of 7 (permalink) Old Jun 5th, 18, 10:26 AM
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Re: proportioning valve AND distribution block ???

Quote:
Originally Posted by Jack Action View Post
That is a proportioning valve. There was none before 1970.
Did 1969 Chevelles have some inherent issue that caused them to add this proportioning valve in 1970? Everything else in the system was the same, right?

I asked this before, and it seems like the responses indicated that the valve is necessary to keep the rear wheels from locking. In the past, I had driven a '69 with OEM discs (Hard - I was 17 yrs old), and I never noticed any real problems with rear wheel lock-up, even under aggressive braking.
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post #6 of 7 (permalink) Old Jun 5th, 18, 11:50 AM
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Re: proportioning valve AND distribution block ???

Quote:
Originally Posted by novadude View Post
Did 1969 Chevelles have some inherent issue that caused them to add this proportioning valve in 1970? Everything else in the system was the same, right?
Yes, everything was the same and I never heard of any issues with pre-70 Chevelles either. And it is to prevent the rear end from locking before the front end (but they should both be able to lock). My guess is that it is a 'fail-safe' feature for an extreme case. Remember that we are post-"Unsafe at Any Speed", which hit GM hard on safety issues.


I can't tell the story about the reasoning behind the prop valve, but I know the one about the metering valve that was used only on disc/drum cars (all of them). It should give you an idea of how far you have to dig to find how decisions are made.


The metering valve restricts the flow to the front brakes until a certain pressure threshold is reached (very low, around 100 psi, maybe less). The reason is to give time for the rear (drum) brakes to apply, as the shoes are retracted by springs and disc pads are not (thus react more quickly).


They are many theories as to why this is needed. None of them convinced me, because mechanically speaking, I can't see no real safety issues of having a few milliseconds response from the rear end. Until I found a theory on the web that made perfect sense to me, from someone that claimed working at GM at the time (don't ask me for a source, I lost it and can't find it again).


The problem was that when front disc brakes appeared, people weren't use to it and, like with everything new, people always have doubts about it. And it was found that some people using the cars with an automatic transmission on icy roads were applying the brakes just enough to secure the front brakes when waiting at red light for example. The result was that the rear end torque was enough to spin the wheels. The solution is just to apply the brakes a little harder, but that left an insecure feeling about this unknown «new technology» of the time for the drivers. So, just because of this very particular case (auto transmission, light braking, on icy road), all cars with front disc brakes were equipped with a metering valve.


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post #7 of 7 (permalink) Old Jun 5th, 18, 1:09 PM
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Re: proportioning valve AND distribution block ???

Thanks, Jack. I guess that makes sense. Interesting about the metering (hold-off) valve.
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