A C3 was not designed with a spring loaded rear brake proportioning valve, and nor does it need one. They did gain one in 1974, and I am sure it had more to do with legal liabilities than any real functional reason. Just because virtually every other car has one, and some lawyer might seize on that. On the 74s+ it would do practically nothing.

Why do I say that? I have raced with and tested adjustable proportioning valves. Once dialed in there is a braking improvement, but only under max braking.

A stock C3 has about 840# on each tire. Under a 1G braking force in a normal car transfers a lot of weight forward. But not so for the corvette. It's center of gravity is so low, that it only 284# transfers forward. Much less than what my Camaro did. The front wheel weights jump to 1125# and the rears drop to 555#. This means 66% of the weight is on the front tires, and the front brakes need to do 66% of the work. Go check the caliper piston sizes, and the front ones have an area that is almost exactly 66% larger than the rear. So they are perfectly balanced. Duntov got it right. Surprise. But only for 1G stops. On a race track. Under any lesser braking force the rears are only doing 33% of the work while they may be carrying much more weight, perhaps even half. But you are so far from the tire's traction limit it does not really matter. 1G was probably slightly more than the race rubber could generate, in 1962. Now pedestrian cars, even Camaros, did not have their brake systems tuned for race track levels of 1G, it was more like .75G, and they desperately need a brake proportioning valve to prevent a spin in panic maneuvers. But a Corvette was tuned for the track. And does not need it.

Now 60 years later, race cars, on race rubber, like slicks, can generate 1.5G on braking, or beyond. 425# would transfer to the front, the rears would drop to 400# each. Now the rear tires have 33% of the brake force, with only 25% of the weight. The rears will lock up first, and the car will spin. This is where you need the adjustable valve. Once you have the car finely balanced for max braking, even a full gas tank, or empty tank, can upset the balance, by adding or taking 150 # of of the rear tires. I decreased my 70mph stopping distance by 30-40ft just by tuning my proportioning valve. But that was a Camaro with a modified braking system that was locking up the rears first, at 1.25G. Most race cars now days even use a cockpit adjustable valve to compensate for heavy/light fuel loads during a race.

I do not understand the use of the spring loaded proportioning valve on the 74s, since the caliper pistons remain the same. There is no reason to cut back on the force to the rear brakes any more than the pistons already do. Those valves are useless. My opinion. Except to a lawyer.

Now on a minivan, with a load capacity that could vary by 1000# in the back, that is a whole 'nuther problem. Or on a race car with it's increased traction. You can cut back the force to the rear, but you can not increase it without changing the calipers and using bigger pistons. Even ABS cars get the basic proportioning pretty close to ideal, under max braking, and then the computer makes fine adjustments for varying tire traction, say in a turn. But in case of ABS failure, it still has to be pretty close to be safe.

End of rant, on these HIGHLY mis-understood Safety Switches, mis-labled on a Corvette as a proportioning valve, when they do not proportion anything, and nor do they need to!

 

Has the OP determined which way the shuttle? valve needs to go? So the correct bleeders can be opened to re-position the valve. Example; you may be bleeding the rear brakes when really you should be bleeding the front brakes or vice-a-versa to get the valve to re-center.

 

It is the owners who mislabel that device as a "proportioning valve". I've not seen that term used in a GM manual for C3 cars.

 

The two terms: Distribution Valve and Proportioning Valve units changed somewhere in the early 70's.
As seen in catalogs, different designs.
Also, the location of the valves were moved from under the master to its new location bolted to the frame down lower, somewhere in early 70's.

Some sellers of those parts do not specify which is which. Other vendors clearly state Proportioning Valve.
But either term will work depending on which model yr you have.

And don't believe everything you read in owners manuals or "some" repair manuals. (Good grief. They are not bibles)
Numerous mistakes that were never corrected before going to print.

 

Anything other than GM manuals [for the most part] are garbage when working on a C3. The Chevrolet Chassis Service Manual, Chevrolet Chassis Overhaul Manual, the Owner's Manual and the Assembly Instruction Manual (AIM) should be all you need to accomplish any task on your C3.