How do I flush, bench test, proportioning valve?
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
How do I flush, bench test, proportioning valve?
I don't want to pay $89 + $12 shippingand wait a few days to get a new Delco unit, so how do I flush and bench test the old one?
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
Re: How do I flush, bench test, proportioning valve? (jsimpson)
The proportioning valve is best tested while on the car. If the valve fails (and the rest of the system is in good working order) then the brake light will come on. If hard pumping on the brakes doesn't shut the light off then the proportioning valve has failed...another indication is the rear wheels locking up during hard braking. Bench testing would require some very expensive equipment.
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (Rockn-Roll)
I already know it's malfunctioning, but about the only thing that could cause it to malfunction is clgging with sludge, so I want to flush it out. Ever tried that?
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Got it out and partially disassembled
I pulled it out; it looks clean inside. One end comes off (the end that the rear brakes are connected to), and there is a spring loaded plunger visible that moves freely. After removing the brake light switch, there is a piston visible that soes NOT move freely. So I'm soaking it in Kroil overnight. I'll play with it again tommorrow.
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Le Mans Master
Joined: Mar 2001
Posts: 6,640
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From: Ann Arbor Michigan
Re: Got it out and partially disassembled (jsimpson)
I'm not sure what "Kroil" is, but if it's a mineral oil based material, you will ruin all the rubber seals inside the valve. What doesn't work? Do the rear brakes lock up or is the brake warning light on? Most all GM shop manuals show a cross section of this valve so you can see how it goes together. There is a very small spring inside the switch piston that pushes the plastic plunger against the spool (the one with the 2 lip seals) and this plastic piston must move freely. If not, the valve will not proportion.
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: Got it out and partially disassembled (Duke94)
The spring loaded pin moves freely. The metal piston does not. The brake light stays on and I never have enough brake pedal to feel comfortable about driving.
Thread Starter
Drifting
Joined: Mar 2000
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (jsimpson)
OK, I sprung for a new proportioning valve today. Art's Corvette Parts in Ft. Lauderdale had one in stock. After disassembling the old one, I came to the conclusion that the only way to adequately bleed the sytem after replacing the proportioning valve is to back bleed the system. So now I'm making some fittings to back bleed with. The fun never ends!
Melting Slicks
Joined: Dec 2000
Posts: 3,407
Likes: 2
From: Carmichael CA
Re: How do I flush, bench test, proportioning valve? (jsimpson)
Back bleed? That's a new one on me. Bleeding using the brake pedal works just fine at the factory when all the parts are new, I don't see why it wouldn't work now. :confused:
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Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (jsimpson)
Backbleeding pushes the bubbles uphill, the direction they are inclined by the laws of physics to travel, rather than downhill.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
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From: Raleigh North Carolina
Re: How do I flush, bench test, proportioning valve? (jsimpson)
It's not a "proportioning valve".... it's simply a switch. So no "proportioning" is done.
When pressure is lost on one side, a differential is sensed, and the piston slides off to one side to block the release of fluid from the master cylinder to the leaky line, and firm up the side that still has back pressure. The small rod going through the top drops down and "grounds", completing the circuit for the brake light.
I have been bleeding these things for a million years, and have never had to "back bleed". But I do understand the concept.
I tend to agree with Rock n Roll.... standard bleeding techniques should work. They have for me.
However.... I normally use gravity bleeding, so I don't accidentally force the piston off to one side while bleeding.
If/when I use the "two man" method, the down strokes are slow & steady.... not hard & fast. This helps avoid the same problem.
When pressure is lost on one side, a differential is sensed, and the piston slides off to one side to block the release of fluid from the master cylinder to the leaky line, and firm up the side that still has back pressure. The small rod going through the top drops down and "grounds", completing the circuit for the brake light.
I have been bleeding these things for a million years, and have never had to "back bleed". But I do understand the concept.
I tend to agree with Rock n Roll.... standard bleeding techniques should work. They have for me.
However.... I normally use gravity bleeding, so I don't accidentally force the piston off to one side while bleeding.
If/when I use the "two man" method, the down strokes are slow & steady.... not hard & fast. This helps avoid the same problem.
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (Tom454)
You are correct; it's not a proportioning valve. Technically, it's a "multipurpose valve". But most aftermarket vendors catalog it asa proportioning valve. Truth be known, the brakes will work just fine if you get rid of it. But it does provide a minute amount of line pressure balancing, for whatever that's worth.
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (jsimpson)
I spent all day garvity bleeding around and around. Used up more than a quart of brake fluid. Now it has enough pedal to get it to a good brake shop and get it pressure bled.
Thread Starter
Drifting
Joined: Mar 2000
Posts: 1,370
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From: Southwest Ranches FL
Re: How do I flush, bench test, proportioning valve? (Tom454)
Tom, that's quite a drive just for a brake bleed! Now if you can build me up an inexpensive TPI for 70 454 while I'm on my way up, THAT would be worth the drive!
