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Discussion Starter #1
I was under my 1989 2500 suburban this past weekend. I was following the rear brake line from the rear axle forward and found that the hard line runs through a wierd valve that is linked to the rear axle housing? Was this some type of early attempt to minimize rear wheel lock up?

I can find any mention of it anywhere. My LMC truck parts catalog pretends it doesn't exist. What is it?

My other question is, can I bypass it? Or is this critical to the safety of the truck's brake system?
 

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The only valve I've seen on my rear axle for the brakes is just a 2-way valve to provide both wheels with braking power, nothing special to speak of....
 

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Discussion Starter #3
Thanks for the reply craw,

This thing is mounted on a bracket coming off the frame rail. The hard brake line then comes off the frame rail, out to this "thing", then back to the frame rail where it continues on to the rear brake hose and splits off to each wheel.

This "thing" then has a 2-piece mechanical link running down to the axle housing. I'll have to get a picture. It is the strangest thing I"ve seen. (not that I've looked at lots of brake systems) But the part of the linkage hooked to the exle housing is just fixed to the housing. The other end (the "thing" end) rotates on the side of this thing.

It really seems that it somehow is monitoring rotation of the axle housing. If the wheels were gripping well during a braking situation, the axle would rotate forward, and the links would then move this rotary valve and continue to adjust brake pressure, and maybe if the rear wheels slip they reduce pressure? Wierd? i don't know.

I just don't want to bypass it if it is critical.
 

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It’s the Fetzer valve serves absolutely no purpose what so ever and cost $300 dollars to buy and $500 in labor to replace.

Sorry I just couldn’t resist, I was kidding.

I have never seen what you are talking about and have been working on vehicles for almost 40 years.

I even went out just now and looked under our 94 Burb that has got to have the stupidest antilock brake system I have ever worked on, just to be sure.

My only suggestion would be to call a dealers parts department and see what they have to say.
 

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I think it is part of your rear abs. It monitors the speed of the internal gears with a magnet kind of like a wheel sensor. Not one of GM's better ideas.
 

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Discussion Starter #6
I think it is part of your rear abs. It monitors the speed of the internal gears with a magnet kind of like a wheel sensor. Not one of GM's better ideas.
96Z71, thanks for the reply.

I don't think my truck has ABS? My brake lines go straight from my master cylinder to the proportioning valve with nothing in between. I don't know about GM ABS, but my ford ABS had a ginormous modulating capitulator (I made that name up) that pulsed the brakes very quickly. And this was located just after the master cylinder and had a tangle of brake lines and electrical wires running in and out of it.

I assumed my truck doesn't have ABS because there was no madulating capitulator thingy.
 

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It's a brake bypass valve. You'll find it on some trucks with a factory trailer package, but not all (dont know why). The bypass valve senses the weight in the cargo area of the truck or on the bumper and adjusts the rear brakes accordingly so you dont have to much or to little braking power from the rear wheels.
Look under a Toyota truck sometime and you'll see the same thing, all Toyota trucks have them.
 

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Discussion Starter #9
Awesome, thanks TB

That makes more sense than my idea of a mechanical anti-lock brake. So should I try to re-use it when re-plumbing the rear brakes? If I bypass it will I have too much rear braking power when I'm not loaded?

How critical is this thing?

Thanks a ton! (actually 3/4 ton!!)
 

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Real critical to the rear brake system. Without it your rear brakes wouldnt work right they'ed be to hard or to soft.
On my 87 Dually I added one and plumbed it to the floor of the cab right in front of the drivers seat. I could adjust the pull of the rear brakes easier when I changed between hauling a trailer and riding empty.
Summit racing sells them they're labeled "Brake Porportional Valves".
 

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Discussion Starter #11
Thanks again TB,

After lots of time on google that is what I've realized too. Without the proportioning valve, my rear brakes would lock up real quick when I was un-loaded.

The idea of a manual adjusting proportioning valve up by the driver's seat is intruiging...I'd love to have that control..but not sure I'm good enough with my double-flares to do so much custom bending and flaring. the line running to the rear brakes goes down the passenger frame-rail, so I'd need to make a serious 90-degree turn and run a line under the passenger seat, over the tranny to the driver's seat area, then back again to the pasenger side frame rail to continue towards the rear wheels.

Assuming the OEM load/height sensing proportioning valves worked o.k., I will probably try to salvage it!
 

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Buy replacement lines, bend, cut, and flare them where you want them to come through the floor at. LMC sells replacement brake lines pre bent.
Or you can replace yours with a manual valve where its at now.
 
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