73 Brake Proportioning Valve

[Kritter]

I have finished installing new SS O-ring calipers, lines, booster, mc, and bled the system. I have peddle but when I start the car and push on the brakes the brake light comes on.

From reading previous postings I believe I need to maintenance the proportioning valve.

I have not been able to search the forum for the previous postings so I am going to ask for help.

Where can I get a rebuild kit for the Proportioning valve - or a new valve?

There isn't much to the valve, and I figure if it is in the kind of shape the original brake parts were, it's toast.

After sitting for 10+ years it is so stuck that I couldn't get it to free up by applying full brake pressure in order to re-set it....I tried.

Any help would be appreciated.

Kritter

 

[69MyWay]

Contact Vette Brakes and Products, and see what they can tell you.

 

[Stallion]

What exactly is this proportioning valve for? What does it do? Where is it located?

Good luck!

TR

 

[JohnZ]

It's not a proportioning valve - it's just a distribution block that contains the differential pressure warning switch; doesn't serve any other function. Vette Brakes and other suppliers have replacements or can rebuild originals.

 

[MaineShark]

What exactly is this proportioning valve for? What does it do? Where is it located?

The proportioning valve adjusts the front/rear brake bias. Since the weight of the car trasnfers forward under braking, the rear brakes have less and less weight on them, the heavier the braking conditions. This makes them more likely to lock-up and skid. The proportioning valve limits the pressure on the rear circuit, to avoid this.

The C3 proportioning valve is sort of interesting, since it varies the proportion, in response to brake pressure. So, if you just tap the brakes, you get more rear brake pressure than if you really jam the pedal hard. This helps to account for the different weight transfer when comparing light braking to heavy braking. Sort of like rudimentiary ABS

And John's correct, it also includes a differential pressure switch, which lights the warming light on the dash, if either circuit loses pressure compared to the other. He's also correct that the '73 has only the warning switch, not a proportioning valve. If I recall correctly, the proportioning valve was added in '74.

As for location, if you follow the lines from the master cylinder, they go right to it. It's mounted on the frame, near the firewall.

Joe

 

[Stallion]

Okay. Thanks for the explanation, guys!

Stallion

 

[Kritter]

MainShark,

You say at first that it provides proportional pressure to front and rear brakes, but then you say it only contains a warning light.

I guess I am wanting to know if there is anything inside that little block that is sticking and causing my front brakes to be tight without pushing on the brakes. I can turn the front tires but they are tight to the point that they won't spin freely with the car raised.

Before I bled the brakes the front wheels would spin freely. I have good peddle, no brake light when car is off (key in acc mode) and applying full brake. When I start the car and apply full brake the brake light comes on.

Is there a spring loaded plunger/piston inside that block?

Kritter

 

[JohnZ]

Yes, there's a brass plunger inside that's held centered by a spring on each side; if the system pressures front-vs-rear change, the plunger shifts toward the weak side and the spool in the center of the plunger touches the switch terminal, grounding it and turning on the warning light (the same light is also connected to the switch on the parking brake handle). If the car has been sitting that long, the internals in the block may have corroded, springs failed, etc.

 

[Kritter]

That's what I was looking for. I thought that is what the configuration was just wanted to make sure.

Thanks to each of you that replied.

I'll keep you posted on my progress.

Kritter

 

[Vettehead Mikey]

Just to clarify, the valve being discussed is not a proportioning valve in any way shape or form. It does not modify the pressure sent to either front or rear brakes. It's sole function is to control the warning switch.

I think some Bubba assumed it was a proportioning valve (which came on later C3s ) and the name unfortunately stuck.

I've seen discussions going on for weeks with people trying to figure out how to 'fix' or adjust it.

Mike

 

[MaineShark]

MainShark,

You say at first that it provides proportional pressure to front and rear brakes, but then you say it only contains a warning light.

Just to clarify, the valve being discussed is not a proportioning valve in any way shape or form. It does not modify the pressure sent to either front or rear brakes. It's sole function is to control the warning switch.

I think some Bubba assumed it was a proportioning valve (which came on later C3s ) and the name unfortunately stuck.

I've seen discussions going on for weeks with people trying to figure out how to 'fix' or adjust it.

The '73 does not have a proportioning valve. Only a pressure warning switch. I'm pretty sure the proportioning valve was introduced in '74. In any case, at some point the change-over was made. Later C3's do have a proportioning valve. And, like I said, it's not just a simple proportioning valve. It has a variable proportioning percentage, which varies in response to line pressure.

Actually, while we're speaking about this critter, there's something you should all be aware of: the internal seals in the pressure warning switch can fail, allowing fluid to cross between the front and rear circuits. Obviously, this negates the safety factor of having split circuits, as all the fluid could escape if any host, fitting, or line ruptures. I believe you can check this by opening a bleed screw on one caliper, and pushing the pedal. It should not go all the way down, because one circuit should still be operational. If it does go all the way to the floor, then that's what you can expect to happen if there is ever damage to your system. If that happens, time for a new valve/switch (depending on year).

Speaking of which, does anyone have an internal diagram? I'm curious if there is some better way to do this, which wouldn't have that risk. I'm thinking some sort of modern electronic pressure sensor, or somesuch, which would have no moving parts which need seals, which can degrade over time.

Joe