Proportioning valve
Racer
Joined: Jan 2007
Posts: 253
Likes: 0
From: Winter Haven Florida
Drifting
Joined: Jan 2006
Posts: 1,944
Likes: 22
From: Yooperville MI
It's a distribution valve technically. It's mounted to the frame below the master cylinder. It's not a quick job because you'll need to bleed the brakes afterwards. The rear line has a couple clamps to the frame behind the valve that need to be disconnected. New GM parts are still plentiful and available for under $100.00, stay away from the rebuilts.
Is yours leaking?
Melting Slicks
Joined: Nov 2001
Posts: 2,668
Likes: 43
From: Eastern US XX
ahoover stated it well.
Proportioning valves are used when the hydraulic pressure requirements between 2 systems [front/rear] are very different - most notably systems with DISC front / DRUM rear brakes. You need to set the proportion [balance] such that one system doesn't grab and subsequently lock up before the other.
Thread Starter
Instructor
Joined: Dec 2000
Posts: 104
Likes: 3
From: Guilford CT
Proportioning valve
had tranny resealed (good neighborhood shop) note on bill that proportioning valve was leaking losing brakes...got one from forum member Gary.G. want to do it myself but am at a loss...
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Actually there is a proportioning valve on 75'. Prior to 74', they just had a "pressure differential valve" which basically is the switch that turns on the red brake light on the dash, The valve is mounted on the frame rail below the master cylinder and looks like this.
Racer
Joined: Jan 2006
Posts: 265
Likes: 0
From: Bowie MD
I am here to cause more trouble and confusion 
Why was a switch made from a differential valve to a proportioning valve? Did GM decide there needed to be a difference in braking pressure to the front/rear disc brakes, or was it done for super extra safety in addition to the two-reservoir master cylinder?
Why was a switch made from a differential valve to a proportioning valve? Did GM decide there needed to be a difference in braking pressure to the front/rear disc brakes, or was it done for super extra safety in addition to the two-reservoir master cylinder?
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Racer
Joined: Jul 2006
Posts: 499
Likes: 0
From: Colebrook Connecticut
thats called a combo valve not a proportioning valve or distribution valve.
basically senses drop in pressure between front and rear brakes.
sends more pressure to the fronts than the rear brakes etc.
its pretty much all the mentioned valves rolled into a single unit.
I was extensively schooled in it just recently.
lol a mechanic set me straight during my disc brake upgrades on my 69 camaro
basically senses drop in pressure between front and rear brakes.
sends more pressure to the fronts than the rear brakes etc.
its pretty much all the mentioned valves rolled into a single unit.
I was extensively schooled in it just recently.
lol a mechanic set me straight during my disc brake upgrades on my 69 camaro
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
I am here to cause more trouble and confusion
Why was a switch made from a differential valve to a proportioning valve? Did GM decide there needed to be a difference in braking pressure to the front/rear disc brakes, or was it done for super extra safety in addition to the two-reservoir master cylinder?
Why was a switch made from a differential valve to a proportioning valve? Did GM decide there needed to be a difference in braking pressure to the front/rear disc brakes, or was it done for super extra safety in addition to the two-reservoir master cylinder?
Racer
Joined: Jan 2007
Posts: 253
Likes: 0
From: Winter Haven Florida
Thanks for the lesson guys I don't think mine needs replacing, but I will assess that when I get to it. I will be replacing everything else, so I probably will do the same with this part. Apparantly it was the only other piece of the brake puzzle that WAS functioning properly. The rear flex lines both had a huge air bubble in them, so when I hit the brakes the Red light came on. I had no idea what it meant at the time, just thought it was telling me I was hitting the brakes, lol.
Thanks again guys,
Josh
I don't think mine needs replacing, but I will assess that when I get to it. I will be replacing everything else, so I probably will do the same with this part. Apparantly it was the only other piece of the brake puzzle that WAS functioning properly. The rear flex lines both had a huge air bubble in them, so when I hit the brakes the Red light came on. I had no idea what it meant at the time, just thought it was telling me I was hitting the brakes, lol.Thanks again guys,
Josh
Le Mans Master
Joined: Oct 2001
Posts: 5,943
Likes: 3
From: Wildwood IL
Valve is one piece brass. Other than it being cracked (Highly doubtfull) or the flares damaged (mabey), the only place it could leak are at the brake line connections. Your trouble might be the bake line fittings not perfectly square/tight to the valve. Very important to get the hard line 90* to the valve flare before tightening. Check there first.
Eddie
Eddie
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Valve is one piece brass. Other than it being cracked (Highly doubtfull) or the flares damaged (mabey), the only place it could leak are at the brake line connections. Your trouble might be the bake line fittings not perfectly square/tight to the valve. Very important to get the hard line 90* to the valve flare before tightening. Check there first.
Eddie
Eddie
Pro
Joined: May 2003
Posts: 601
Likes: 0
From: Nottingham
Interested in this at the moment, as I've just had the combination valve apart on my 80, as part of a brake system refurb...
Duke94, it's essentially identical to the one you have pictured.
Now, correct me if I'm wrong, but to my understanding this valve provides no proportioning function, but simply acts as a differential pressure sensor, ie it turns the red light on when pressure is lost in either front or back sytems.
Is the braking force not "proportioned" simply by using smaller pistons in the rears?
There may be something I'm missing here but I can't see how the pictured valve /mine can provide any difference in pressure between front and rear systems.....
Duke94, it's essentially identical to the one you have pictured.
Now, correct me if I'm wrong, but to my understanding this valve provides no proportioning function, but simply acts as a differential pressure sensor, ie it turns the red light on when pressure is lost in either front or back sytems.
Is the braking force not "proportioned" simply by using smaller pistons in the rears?
There may be something I'm missing here but I can't see how the pictured valve /mine can provide any difference in pressure between front and rear systems.....
Le Mans Master
Joined: Mar 2001
Posts: 6,640
Likes: 287
From: Ann Arbor Michigan
Interested in this at the moment, as I've just had the combination valve apart on my 80, as part of a brake system refurb...
Duke94, it's essentially identical to the one you have pictured.
Now, correct me if I'm wrong, but to my understanding this valve provides no proportioning function, but simply acts as a differential pressure sensor, ie it turns the red light on when pressure is lost in either front or back sytems.
Is the braking force not "proportioned" simply by using smaller pistons in the rears?
There may be something I'm missing here but I can't see how the pictured valve /mine can provide any difference in pressure between front and rear systems.....
Duke94, it's essentially identical to the one you have pictured.
Now, correct me if I'm wrong, but to my understanding this valve provides no proportioning function, but simply acts as a differential pressure sensor, ie it turns the red light on when pressure is lost in either front or back sytems.
Is the braking force not "proportioned" simply by using smaller pistons in the rears?
There may be something I'm missing here but I can't see how the pictured valve /mine can provide any difference in pressure between front and rear systems.....
Pro
Joined: May 2003
Posts: 601
Likes: 0
From: Nottingham
Aha! Yep, I was missing something 
I rather naively assumed that there was little of interest inside the end plug, and that it was just some form of extension adapter or something.
From the parts I saw in front of me, I couldn't work out how there could possibly be any proportioning action when there were no springs to create the proportioning effect. Seeing that makes a lot more sense, thanks for the picture. Yet again the forum's timing is spot-on!
How does the end of that plug come apart? I saw there are two smaller holes either side of the main hole (on the inner face) - do you put a pin-spanner in those and turn, or is there some other method?
I'll have to go and have another good look at it now
cheers!
Theo
I rather naively assumed that there was little of interest inside the end plug, and that it was just some form of extension adapter or something.
From the parts I saw in front of me, I couldn't work out how there could possibly be any proportioning action when there were no springs to create the proportioning effect. Seeing that makes a lot more sense, thanks for the picture. Yet again the forum's timing is spot-on!
How does the end of that plug come apart? I saw there are two smaller holes either side of the main hole (on the inner face) - do you put a pin-spanner in those and turn, or is there some other method?
I'll have to go and have another good look at it now
cheers!
Theo