dual rear brake line proportioning valve 1986 RS V6 - First Generation Forum

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dual rear brake line proportioning valve 1986 RS V6
Tuesday, November 10, 2020 5:22 PM
A C Online
Any advice for what is the original part number or an equivalent replacement for the fender mounted brake proportioning valve on a 1986 Cavalier RS convertible with 2.8L V6?
It seems to have 6 bubble flare ports, 1 mounting hole, and an electrical connector.
The bottom 2 ports are for separate right side and left side brake lines to the rear drum brakes.
The top 2 ports are the inputs from the master cylinder.
And 2 middle ports for the front disc brakes.

Re: dual rear brake line proportioning valve 1986 RS V6
Wednesday, November 11, 2020 12:42 AM
I think this might be what you are looking for:

VALVE , BRK COMB VLV (1985 - 85).....25519477
Re: dual rear brake line proportioning valve 1986 RS V6
Wednesday, November 11, 2020 12:44 AM
butch nackley wrote:

I think this might be what you are looking for:

VALVE , BRK COMB VLV (1985 - 85).....25519477

Sorry, too early in the morning.
(1985 - 86)
Re: dual rear brake line proportioning valve 1986 RS V6
Wednesday, November 11, 2020 8:06 AM
A C Online
I can read 44 KH 436 6 stamped into the side. The bottom two ports are for the rear drums. The top two are from the master cylinder. The front and back ports seem to be for the front calipers. On the rear port I see what seems to be an extra fitting. And this has just 1 mounting hole.

Thanks Butch Nackley for the information. Do you have any link to a source for the part? Or some idea where that part number you have can be searched?
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Re: dual rear brake line proportioning valve 1986 RS V6
Wednesday, November 11, 2020 4:13 PM
That looks like what the drawing in the parts book shows. And it shows it is attached with one bolt.
You can contact this place; http://www.kakapart.com/parts/gm-25519477.html
I idea if they actually have it or not, but maybe worth a try.
Re: dual rear brake line proportioning valve 1986 RS V6
Today 3:30 PM
A C Online
I am not able see any picture at that kakapart link, or anywhere else.
Having studied this car's brake parts a bit more, it seems that there are two diagonal paths, one using fine thread ISO/Bubble Flare connectors and the other using coarse thread.
The one diagonal path is front right caliper and rear left drum. And vice versa.
All the fine thread ports seem to be 12mm, except for the one for the rear right, which is 10mm.
And that port is now quite stripped out on my original piece.
I am not sure if there is enough metal from the block itself to safely tap it out to 12mm.

Running through this box there seem to be 2 or 3 pieces screwed in most of the width of the aluminum block.
[Looking into the ports there is some visible brass inside.]
And there is an electrical connector.
I am assuming that GM designed it to have two different proportioning valves screwed in, so that each diagonal has its own pressure division.
The 3rd piece seems to be something else and might have something to do with the electrical switch, but it also seems to have a slightly special output port for I think the front left caliper (it could also be the right one, but since it is a 12mm fine thread, I deduce it must be the front left, since I'm focused on the rear right.)

I'd very much appreciate knowing more about what is going on, and what options I might have if I need to convert this over to a different brake system.

I suspect the 4-cyl version of this same car uses a different proportioning valve system, which is not fender mounted. Is it the same master cylinder, with one coarse thread and one fine thread? Does the 4-cyl version have a common line for both rear drum brakes? Would it even be the same proportioning valve, just in a different distribution block style?

While I once helped convert a '69 GM car from 4 drums to front disc I've never really given much thought to proportioning valves before.

Since my car's somewhat unique system was only in use for 2 years I suspect that (like it's 2.8L V6) it wasn't GM's best design, and that the diagonal braking pattern doesn't really provide much advantage over a more traditional plumbing pattern.
I'd like to better understand so that I can keep this car as safe as it was designed to be.
Higher quality brake lines that are less susceptible to rusting might be enough that I don't particularly need the fancy diagonal pattern that I suspect is supposed to keep 3 out of 4 brakes working if any one of the 4 independent brake lines break.
What other factors go into mid '80s J-Body brake system designs and safety considerations?
Re: dual rear brake line proportioning valve 1986 RS V6
Today 5:10 PM
A C Online
I'm not able see any picture at that kakapart link.
Having studied this car's brake parts a bit more, it seems that there are two diagonal paths, one using fine thread ISO/Bubble Flare connectors and the other using coarse thread.
The one diagonal path is front right caliper and rear left drum. And vice versa.
All the fine thread ports seem to be 12mm, except for the one for the rear right, which is 10mm.
And that port is now quite stripped out on my original piece.
I am not sure if there is enough metal to safely tap it out to 12mm.

Running through this box there seem to be 2 or 3 pieces screwed in most of the width of the aluminum block.
[Looking into the ports there is some visible brass inside.]
And there is an electrical connector.
I am assuming that GM designed it to have two different proportioning valves screwed in, so that each diagonal has its own pressure division.
The 3rd piece seems to be something else and might have something to do with the electrical switch, but it also seems to have a slightly special output port for I think the front right caliper (it could also be the left one, but since it is a 12mm fine thread, I deduce it must be the front right.)

I'd very much appreciate knowing more about what is going on, and what options I might have if I need to convert this over to a different brake system.

I suspect the 4-cyl version of this same car uses a different proportioning valve system, that is not fender mounted. Is it the same master cylinder, with one coarse thread and one fine thread? Does it have a common line for both rear drum brakes? Would it even be the same proportioning valve, just in a different distribution block style?

While I once helped convert a '69 GM car from 4 drums to front disc I've never really given much thought to proportioning valves before.
Since my car's somewhat unique system was only in use for 2 years I suspect that (like the 2.8L V6) it wasn't GM's best design, and that the diagonal braking pattern doesn't really provide much advantage over a more traditional plumbing pattern.
I'd like to better understand so that I can keep this car as safe as it was designed to be.
Higher quality brake lines that are less susceptible to rusting might be enough that I don't particularly need the fancy diagonal pattern that I suspect is supposed to keep 3 out of 4 brakes working if any one of the 4 independent brake lines break.
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