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post #1 of (permalink) Old 01-30-2004, 10:02 AM Thread Starter
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Proportioning valve

In a couple of recent threads, people are suspecting a bad proportioning valve causing them to have braking problems. I've found that the "mystery part" always gets the blame. I've seen it many times. I've claimed that if a black box were added to an electrical panel with a bunch of wires coming out of it but not connected to anything, anywhere, most people would blame it for every problem that ever occurred.

I found this article on the web about proportioning valves. It's got a couple problems in the write-up, like saying that the braking force changes with drum width. It's the diameter of the drum and the cross sectional area of the piston that gives more or less brake force. The width just helps with spreading the frictional force out to reduce the heat. It does touch on the issue of drums providing more braking force than discs and that the valve keeps the rear drums from locking up before the front discs.

This is not a magic part and it's the last thing I would expect to be the problem although I'm sure one went bad somewhere once. Unless it's leaking, I'd look somewhere else. If you have a spongy pedal, you have air in the system. If you have good pedal but the pedal slowly goes to the floor, you have a leak.

Here's a cutaway drawing of a proportioning valve.

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post #2 of (permalink) Old 01-30-2004, 10:11 AM
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Re: Proportioning valve

1. Ignorance is Bliss.
2. Now I'm a little unhappier,
3. But on the other hand...
4. Knowledge is Power!
5. Thanks for the link!
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post #3 of (permalink) Old 01-30-2004, 04:33 PM
 
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Re: Proportioning valve

Very good information. The replacement of a stock proportioning valve with an aftermarket one sounds like a far too common error.
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post #4 of (permalink) Old 01-31-2004, 01:27 PM
 
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Re: Proportioning valve

"I found this article on the web about proportioning valves. It's got a couple problems in the write-up, like saying that the braking force changes with drum width."

====


Actually it has a more basic problem then the one you pointed out. Its title...and the cut away view. The valve being described and pictured is a Combination Valve, of which the proportioning valve is one (section or fuction) of the combination valve... just a point of clarafication...09
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post #5 of (permalink) Old 01-31-2004, 04:08 PM Thread Starter
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Re: Proportioning valve

Actually, it is commonly referred to as a proportioning valve even though it has a combination of functions as stated in the article and as stated in the text under the cutaway figure. To me, a combination valve is a big heavy piece of equipment in a diesel locomotive cab that allows the engineer to either independently apply the locomotive (head end) brakes or apply brakes to the entire train through the trainline. I'm sure that there are other "combination valves" so, to avoid causing confusing the counterman at Autozone, it has been referred to as a proportioning valve.
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