Proportioning Valve
Thread Starter
Heel & Toe
Joined: Feb 2013
Posts: 20
Likes: 0
Proportioning Valve
Hi, are proportioning valves restorable? My 71 brake light comes on when braking hard. It actually feels like I'm only getting braking power at the rear. It could be the valve or brake hoses up front may have collapsed (easy fix). The thing is the valves are date coded and I would prefer repairing it instead of buying a new one.
Thank you,
Ydubbs
Thank you,
Ydubbs
Race Director
Joined: May 2006
Posts: 18,734
Likes: 2,582
here's a thread on the question but you may want to check for leaks first
http://forums.corvetteforum.com/fact...e-rebuild.html
http://forums.corvetteforum.com/fact...e-rebuild.html
Pro
Joined: Feb 2000
Posts: 741
Likes: 46
From: Hebron IN
Mine developed a leak out of a port. All the corvette specific dealers wanted an insane amount for them and iirc they were rebuilt units, so apparently it can be done.
I ended up going to my local Chevy dealer and they had a brand new one for me the next day for about 1/5 the price. Looked 100% identical to the original.
Not sure about numbers...when it comes to something as important as brakes, the microscopic print on an item that fits in the palm of my hand and is mounted under a car means absolutely nothing to me.
Old part rebuilt by who knows for $250 or brand new factory GM for $50.
I ended up going to my local Chevy dealer and they had a brand new one for me the next day for about 1/5 the price. Looked 100% identical to the original.
Not sure about numbers...when it comes to something as important as brakes, the microscopic print on an item that fits in the palm of my hand and is mounted under a car means absolutely nothing to me.
Old part rebuilt by who knows for $250 or brand new factory GM for $50.
Burning Brakes
Joined: Aug 2004
Posts: 802
Likes: 1
They are easy to take apart and are very simple inside. I took mine apart for inspection. It`s basically a plunger with a spring on each side that centers the plunger. When the pressure is not equal, the plunger moves and contacts a finger and creates a ground. The plunger has an o ring on each end.
If there is no corrosion inside and the springs not broken or week, the most that needs to be done is new o-rings.
Steve L
73 coupe since new
If there is no corrosion inside and the springs not broken or week, the most that needs to be done is new o-rings.
Steve L
73 coupe since new
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
That unit is not really a "proportioning valve", but that's what folks call it. That unit is really just a shuttle valve (formal name, "combination valve") that is supposed to 'trip' whenever a leak develops in the front OR the rear section of the brake system. With a leak, only the 'good' side develops pressure, so the shuttle valve trips and blocks the brake fluid from reaching the section with the leak.
However, you can also 'trip' that shuttle valve if you bleed the brakes and depress the brake pedal rapidly when doing so.
The Chevrolet Chassis Service Manual describes the use of a special tool to keep that valve in neutral position when bleeding the brakes. But, it does not say how to reset it once the valve has tripped.
Hopefully, others responding to this thread can provide that info.
However, you can also 'trip' that shuttle valve if you bleed the brakes and depress the brake pedal rapidly when doing so.
The Chevrolet Chassis Service Manual describes the use of a special tool to keep that valve in neutral position when bleeding the brakes. But, it does not say how to reset it once the valve has tripped.
Hopefully, others responding to this thread can provide that info.
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Burning Brakes
Joined: Aug 2004
Posts: 802
Likes: 1
That unit is not really a "proportioning valve", but that's what folks call it. That unit is really just a shuttle valve (formal name, "combination valve") that is supposed to 'trip' whenever a leak develops in the front OR the rear section of the brake system. With a leak, only the 'good' side develops pressure, so the shuttle valve trips and blocks the brake fluid from reaching the section with the leak.
However, you can also 'trip' that shuttle valve if you bleed the brakes and depress the brake pedal rapidly when doing so.
The Chevrolet Chassis Service Manual describes the use of a special tool to keep that valve in neutral position when bleeding the brakes. But, it does not say how to reset it once the valve has tripped.
Hopefully, others responding to this thread can provide that info.
However, you can also 'trip' that shuttle valve if you bleed the brakes and depress the brake pedal rapidly when doing so.
The Chevrolet Chassis Service Manual describes the use of a special tool to keep that valve in neutral position when bleeding the brakes. But, it does not say how to reset it once the valve has tripped.
Hopefully, others responding to this thread can provide that info.
I probably would have noticed it,,,, but I have been wrong before.
Steve L
72 coupe since new
Pro
Joined: Feb 2000
Posts: 741
Likes: 46
From: Hebron IN
The '73 and earlier must be completely different than the later models. My '75 definitely had a proportioning valve which is even what GM calls it. Actually it was a combination valve as it also contains the switch for the warning light. Resetting the switch is simply done by pressing really hard on the brake pedal.
A small port with a rubber plug on mine ruptured which would slowly leak fluid and suck in air over time.
This is pretty much it:
https://www.musclecarresearch.com/gm-1257208-rebuild
The valve never blocked off a failed side or impeded brake bleeding. With a ruptured line, you just dumped fluid with each pedal press while the other MC reservoir maintained a portion of braking.
Now I'm curious...how did GM control the proportioning on the earlier C3s? You can't have equal pressure front and rear without turning the car into a death trap, especially on wet roads. So how did they handle it?
A small port with a rubber plug on mine ruptured which would slowly leak fluid and suck in air over time.
This is pretty much it:
https://www.musclecarresearch.com/gm-1257208-rebuild
The valve never blocked off a failed side or impeded brake bleeding. With a ruptured line, you just dumped fluid with each pedal press while the other MC reservoir maintained a portion of braking.
Now I'm curious...how did GM control the proportioning on the earlier C3s? You can't have equal pressure front and rear without turning the car into a death trap, especially on wet roads. So how did they handle it?
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Diameter of pistons in the caliper. Much smaller in the rear.
Thread Starter
Heel & Toe
Joined: Feb 2013
Posts: 20
Likes: 0
Thank you for all the feedback and input. I had bled the brakes in the spring post new master cylinder install. Light came on soon there after. Perhaps the piston wasn't balanced correctly during that process? There are no leaks throughout, fluid in master is full after driving periodically throughout the summer.
