Proportioning Valve Removal
Race Director
Joined: Apr 2000
Posts: 17,676
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Well it does something GM won't spend 1 cent unless it has to. Perhaps allows the car to be stopped if the either one of the fronts leaks all the fluid out or the back. This triggers the light and a mechanism inside to give you stopping capability, not great but at least you can stop.
I believe the correct name for it is Distribution Block, a Proportioning Valve will have a **** with numbers so you can set it to brake more to the front or back. You will see them on road race cars.
Anyway it happened to me this summer blew the O ring between the halves of the front passenger side and a nice smoke show for a few miles but I was still able to get home.
Now I fixed the caliper and bled the brakes but the stupid brake light won't go out , I drove around a bit and hammered the brakes and the fronts just locked up and the light stayed on, anyone know how to make it go out ?
I believe the correct name for it is Distribution Block, a Proportioning Valve will have a **** with numbers so you can set it to brake more to the front or back. You will see them on road race cars.
Anyway it happened to me this summer blew the O ring between the halves of the front passenger side and a nice smoke show for a few miles but I was still able to get home.
Now I fixed the caliper and bled the brakes but the stupid brake light won't go out , I drove around a bit and hammered the brakes and the fronts just locked up and the light stayed on, anyone know how to make it go out ?
Last edited by MotorHead; Dec 30, 2011 at 06:43 PM.
Race Director
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I'm also pretty confused and unsure which years beyond '73 did and did not actually have a proportioning function inside the distribution block.
Somehow I thought it was when the back end changed in '78. Dunno.
Team Owner
Joined: Aug 1999
Posts: 30,995
Likes: 98
From: Ontario
I'm not doubting what you said or the links posted, just that Paragon (and other vendors) use incorrect terminology for the '70-'73 part.
I'm also pretty confused and unsure which years beyond '73 did and did not actually have a proportioning function inside the distribution block. Somehow I thought it was when the back end changed in '78. Dunno.
I'm also pretty confused and unsure which years beyond '73 did and did not actually have a proportioning function inside the distribution block.
Somehow I thought it was when the back end changed in '78. Dunno.
Melting Slicks
Joined: Aug 2001
Posts: 3,049
Likes: 124
From: Wichita KS
I'm not doubting what you said or the links posted, just that Paragon (and other vendors) use incorrect terminology for the '70-'73 part.
I'm also pretty confused and unsure which years beyond '73 did and did not actually have a proportioning function inside the distribution block. Somehow I thought it was when the back end changed in '78. Dunno.
I'm also pretty confused and unsure which years beyond '73 did and did not actually have a proportioning function inside the distribution block.
Somehow I thought it was when the back end changed in '78. Dunno.George McNicholl in his book, "How to Rebuild Corvette Rolling Chassis 1963-1982" (p131) notes:
GM 3904303 rear brake pressure-regulator switch is installed in 1967 and 1968 Corvettes with GM bracket 3910797 bracket.
GM 3956787 rear brake pressure-regulator switch is installed in 1969.
GM 3980797 rear brake pressure-regulator switch is installed in 1970-1973.
1974-1977 used 339780, and 1978-1982 used 1257208.
GM 3878944, an adjustable proportioning valve, was only used on 1966-1970 Corvettes equipped with J50 power brake and J56 heavy-duty brake options.
Last edited by mapman; Dec 30, 2011 at 08:13 PM.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Melting Slicks
Joined: Aug 2001
Posts: 3,049
Likes: 124
From: Wichita KS
Well it does something GM won't spend 1 cent unless it has to. Perhaps allows the car to be stopped if the either one of the fronts leaks all the fluid out or the back. This triggers the light and a mechanism inside to give you stopping capability, not great but at least you can stop.
I believe the correct name for it is Distribution Block, a Proportioning Valve will have a **** with numbers so you can set it to brake more to the front or back. You will see them on road race cars.
Anyway it happened to me this summer blew the O ring between the halves of the front passenger side and a nice smoke show for a few miles but I was still able to get home.
Now I fixed the caliper and bled the brakes but the stupid brake light won't go out , I drove around a bit and hammered the brakes and the fronts just locked up and the light stayed on, anyone know how to make it go out ?
I believe the correct name for it is Distribution Block, a Proportioning Valve will have a **** with numbers so you can set it to brake more to the front or back. You will see them on road race cars.
Anyway it happened to me this summer blew the O ring between the halves of the front passenger side and a nice smoke show for a few miles but I was still able to get home.
Now I fixed the caliper and bled the brakes but the stupid brake light won't go out , I drove around a bit and hammered the brakes and the fronts just locked up and the light stayed on, anyone know how to make it go out ?
Perhaps your switch is damaged.
For 1980, the shop manual (p 5-13) indicates the switch is NOT serviced. If it fails or is damaged, the combination valve is replaced.
The 1980 shop manual (p 5-2) indicates the switch needs a pedal force of 450 psi to reset (after the repair is made).
Melting Slicks
Joined: Aug 2001
Posts: 3,049
Likes: 124
From: Wichita KS
The 1953-1982 Corvette Parts and Illustration Catalog (Group 4.690) narrative for the 3980797 part number is Switch-Valve, Brake Pressure Regulator. I would NOT expect any pressure regulating hardware inside the block - most likely they just made the orifice feeding the rear brakes slightly smaller than the hole feeding the front brakes.
Last edited by mapman; Dec 30, 2011 at 09:01 PM.
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Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
The block is a single assembly, COMBINATION, failure warning switch and proportioner, that also serves as the junction block. I think the only adjustable proportioning valves were on 1966-1970 RPO J50/J56.
Perhaps your switch is damaged.
For 1980, the shop manual (p 5-13) indicates the switch is NOT serviced. If it fails or is damaged, the combination valve is replaced.
The 1980 shop manual (p 5-2) indicates the switch needs a pedal force of 450 psi to reset (after the repair is made).
Perhaps your switch is damaged.
For 1980, the shop manual (p 5-13) indicates the switch is NOT serviced. If it fails or is damaged, the combination valve is replaced.
The 1980 shop manual (p 5-2) indicates the switch needs a pedal force of 450 psi to reset (after the repair is made).
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
The 1953-1982 Corvette Parts and Illustration Catalog (Group 4.690) narrative for the 3980797 part number is Switch-Valve, Brake Pressure Regulator. I would NOT expect any pressure regulating hardware inside the block - most likely they just made the orifice feeding the rear brakes slightly smaller than the hole feeding the front brakes.
Melting Slicks
Joined: Aug 2001
Posts: 3,049
Likes: 124
From: Wichita KS
Last edited by mapman; Dec 31, 2011 at 03:45 PM.
Race Director
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That's present in the '74? and up valves, not prior versions. As I understand the function. the valve works by first ensuring that the rear brakes are applied, then the fronts.
Size of the lines or orifices do not control pressure, only flow.
Here's a post of interest:
http://www.vettemod.com/forum/showthread.php?t=5706
Size of the lines or orifices do not control pressure, only flow.
Here's a post of interest:
http://www.vettemod.com/forum/showthread.php?t=5706
Last edited by Mike Ward; Dec 31, 2011 at 10:30 AM. Reason: meant to say '74? and up valves. not '73
Pro
Joined: May 2011
Posts: 581
Likes: 5
From: BC
That's present in the '73? and up valves, not prior versions. As I understand the function. the valve works by first ensuring that the rear brakes are applied, then the fronts.
Size of the lines or orifices do not control pressure, only flow.
Here's a post of interest:
http://www.vettemod.com/forum/showthread.php?t=5706
Size of the lines or orifices do not control pressure, only flow.
Here's a post of interest:
http://www.vettemod.com/forum/showthread.php?t=5706
Steve g
Pro
Joined: May 2011
Posts: 581
Likes: 5
From: BC
Yeah thanks, gonna out and stand on it. I believe I had this problem about 10 years ago and took the whole block out and reset it manually I do not want to do that again it was also replaced with a new one 2 years ago when I did my entire braking system. For some reason it is not getting enough pressure, the hydroboost certainly puts out more than 450psi
loosen a line fitting on either the frt or the rear (at the MC). Slowly increase the pedal pressure until the light goes off. If you reach max pedal pressure and it hasn't gone of close that fitting and loosen a fitting in the other circuit and repeat. This applies pressure on one end of the spool only and will force it over. Because we don't know what side it's sitting on we try one and if it doesn't work we move it the other way. If it goes off then comes on again you went to far and now have to open the other side to move it back. The idea is to push the pedal until the light goes off and release it right away and tighten the line.
Steve g
Race Director
Joined: Mar 2006
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From: Florida
Other way around, frt brakes first. There is nothing in the front brake line so regardless of what's inside the proportioning valve, it is inline to the rears and as such,unless it's a pump, it won't make the rears apply first. I believe it acts as a pressure regulator balancing a combination of hyd pressure and spring pressure against line pressure.
Steve g
Steve g
First, it works primarily on pressure not flow. (hydraulics can be confusing to some because there is never one without the other and portions thereof)
The greatest confusion arises from disc/drum, there is a metering valve that limits the front at very low pressures until the rear drums start to work, then both work together.
Corvette disc systems don't use a metering valve.
The proportioning valve reduces the rear disc pressure and increases at a reduced rate than the front with harder breaking, once it has been triggered beyond the kick point. It's last purpose is to allow full pressure to the rear if triggered by the differential valve spool showing a front system pressure loss.
Front caliper piston area is larger than rear, so the earlier cars with no prop valve had that amount of basic bias built in, thus it also never changed with higher pressures.
450 psi is only a moderate brake application. A hard application is about 1500 psi, which is the test pressure used. If it hasn't reset with a few hard applications it's likely stuck. You can take it apart and clean it and replace the o-rings or:
loosen a line fitting on either the frt or the rear (at the MC). Slowly increase the pedal pressure until the light goes off. If you reach max pedal pressure and it hasn't gone of close that fitting and loosen a fitting in the other circuit and repeat. This applies pressure on one end of the spool only and will force it over. Because we don't know what side it's sitting on we try one and if it doesn't work we move it the other way. If it goes off then comes on again you went to far and now have to open the other side to move it back. The idea is to push the pedal until the light goes off and release it right away and tighten the line.
Steve g
loosen a line fitting on either the frt or the rear (at the MC). Slowly increase the pedal pressure until the light goes off. If you reach max pedal pressure and it hasn't gone of close that fitting and loosen a fitting in the other circuit and repeat. This applies pressure on one end of the spool only and will force it over. Because we don't know what side it's sitting on we try one and if it doesn't work we move it the other way. If it goes off then comes on again you went to far and now have to open the other side to move it back. The idea is to push the pedal until the light goes off and release it right away and tighten the line.
Steve g
If I may add a little more. Less messy to use just the bleeder screws and hoses into a jug. Time well spent to take these valves apart and clean them, if your system was really cruddy.
BTW, I believe it's Duke94 (if I have it right), that is the last word on these valves.
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
Yeah thanks, gonna out and stand on it. I believe I had this problem about 10 years ago and took the whole block out and reset it manually I do not want to do that again it was also replaced with a new one 2 years ago when I did my entire braking system. For some reason it is not getting enough pressure, the hydroboost certainly puts out more than 450psi
Oddly enough I just replaced mine on my 74 yesterday. Took it down the road and the effin light came on. Tried all the stompin in the world and it didnt matter.
Took it home and wouldnt you know it, but one of the fittings was loose enough to leak. Tightened it, and now seems to be dandy.
Race Director
Joined: Jul 2001
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And there's the key- the built-in design bias was 'set' for the '65 model year when disk brakes first appeared and not changed at all irrespective of SBC or BBC versions, or when the C3s came out in '68. If the prop valve first appeared in '74 (thinking that Paul and aussiejohn are correct), I wonder what prompted GM to take this tack instead of resetting the design bias?
Pro
Joined: May 2011
Posts: 581
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From: BC
And there's the key- the built-in design bias was 'set' for the '65 model year when disk brakes first appeared and not changed at all irrespective of SBC or BBC versions, or when the C3s came out in '68. If the prop valve first appeared in '74 (thinking that Paul and aussiejohn are correct), I wonder what prompted GM to take this tack instead of resetting the design bias?
Steve g
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Its not so much standing on it, as it is a kick/stomp to reset the switch. Give it a good swift kick in the ***!
Oddly enough I just replaced mine on my 74 yesterday. Took it down the road and the effin light came on. Tried all the stompin in the world and it didnt matter.
Took it home and wouldnt you know it, but one of the fittings was loose enough to leak. Tightened it, and now seems to be dandy.
Oddly enough I just replaced mine on my 74 yesterday. Took it down the road and the effin light came on. Tried all the stompin in the world and it didnt matter.
Took it home and wouldnt you know it, but one of the fittings was loose enough to leak. Tightened it, and now seems to be dandy.
Yeah that's what I did a few times, the hydroboost IIRC puts out 1800psi, I like the idea of loosening the fittings, thanks
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
Apparently I jinxed myself, as I just took it out for a ride and Ive got the redlight back on here and there, and it appears that the brakeline out of the prop valve to the rear brakes may be leaking....Im sure after I fix that, I will discover a new leak and then another one etc...sometimes I hate myself for buying this car