where is this???

 

On the frame under the master cylinder, although it's not really a proportioning valve. It has a brake failure warning switch in it.

 

pics anyone?

 

Type 'c3 corvette brake proportioning valve' into google, and then select images. All sorts of pictures there...

 

It's my understanding the '68 and 78-82 had proportioning valves in their system.

Correct me if I'm wrong.

 

I'm not certain enough to argue it Mike, I would be interested in the facts and rationale behind the change though. As the calipers and rotors as well as the master cylinder size is the same for the C3's. Could it be that the heavier 78-82's needed the proportion valve due to the weight?

 

All 68 to 82 cars had a proportioning valve.. and all had a low pressure switch to toggle the "brake" lamp in the dash if the pressure was too low. Basically the system is the same in function but differed in line size over the production run. You had five different valves for this run.. and a few different switches.

1967-1968
1969
1970-1973
1974-1977
1978-1982

All valves were located next to the lower frame section below the master cylinder.


Brake valve switches.

 

I feel I must demur. My 69 has identical pressures, as measured personally by me,, front and back. Given the rather large disparity between the piston area front to back, further proportioning would seem to make the rears a more useless appendage than they already are. I once computed the weight transfer and came nearly dead on to the same proportion as the brake area designed in by Chevy. The only reason I could think for further reduction in rear brake performance would be for big block cars.

 

All I stated was that they all have proportioning valves and low pressure switches.. I made no mention of the proportioning effects of each valve front to rear or their restricted ratio's because honestly I don't know them. All I roll by is what GM delivered and what was deemed correct by the GM design teams for the specified years listed above.

People replace these valves and why... Well I don't have a clue because aside from the switch going bad, this is just a glorified brake distribution block that can clog up and be cleaned out very easily.

I have the original essential GM pressure sensor tool system where you put sensors on the pads to detect the pressure.. but in all my years I've never seen a reason to use it. This system will tell you the pressure at each caliper but I've always deemed it a waste of time....

Willcox

 

I think what is inside that brass block is some sort of sliding piston centered by springs. If there is a pressure loss on one side or the other the piston covers or uncovers the plunger on the switches you illustrated and the light on the console goes on to highlight the difference in pressure. I have an old one that I just looked at but I can't for the life of me see how to tell what's inside. And my electrical connection hex nut looks a bit rounded so I'm guessing that's why I have a new one installed. I believe "proportioning valve" misrepresents what this brass block does, at least to some of us.

 

I can tell that on a 77 the switch function is as described above, but there is also a fitting for the rear brake lines that is mounted on the back of the "brass block", and it may somehow reduce the rear brake effectivity???? i.e. proportioning. BUT, I am just guessing.

I also don't try to second guess GM engineering. I disassembled it, cleaned it, put new O-rings on it, and put it back on the car. The brakes do what they need to do.

 

Correct. All C3s had a brake pressure differential switch to detect an imbalance between the front and rear brakes. The optional J-56 Heavy Duty Brake package for 68 and 69 had a proportioning valve in addition to the differential switch.

 

Wow great information and I found it cleaned it off and used what I read in one of the forms about opening the front bleeder pump pressure up and the brake light went off it worked for now LOL the brake pedal still is not as hard while driving as I would like but I guess that's just the way it is

 

I agree, but the question still remains about the 78-82 units. I've seen some internal drawings of those and there's a proportioning piston inside near where the rear brake line attaches.

 

Wasnt the proportional valve on the l88 type cars user adjustable where the ones on the later c3s just set at one point of proportioning

 

I believe you're correct.

 

ok.. here is the scoop, later years had a proportioning valve earlier years it was just a distribution block... ether way, it is made in such a way that your rear brakes will never lock up.. the reason? $$$$ and safety, if your rears lock up, it is very easy for the back to come up front! No fun.... so to protect GM and the costumer, Gm from getting sued and us from getting killed, they made it this way.. you are only getting about 30%-70% max use of your rear brakes. and everyone is happy.. except people like me who want to competitively race there C3's up agents C4,5,6 and 7's .... Well the brakes are going to have to be modified to do this, and the back brakes need to do there part to help stop this car.... so., what to do you ask?

Wildwood makes a KneeBrake adjustable proportioning valve.. What is this you ask? Well, let me try and explain how it works... The valve gives a 50/50 split between the front and back.. which now intern starts really using those back brakes.. until you get to the lock up part, then 50/50 is BAD... in comes the "adjustable" part... by adjusting the proportioning valve what you are doing is setting the knee brake point.. here is how that works. lets say (just numbers for an example) you start to put pressure on the brakes, front and back will have the exact same pressure until... until you get to the adjustment point you set, which should be JUST BEFORE lockup of the rear brakes. Ahh at this point the "knee brake" happens, and the front continue going up in pressure in a linear path, but the back brakes now, from this pressure point forward, only get 50% of on going pressure...

maybe this would help more.. Knee brake set at 75... so from 0-75 front and rear are equal, at 77 the front is 77 and the rear is 76, 50% difference after the set 75 point... so at say 95, the front would be 95 and the rear would be 85 (75+(20/2)... this keeps your back brakes from locking up, and at the same time gives much more stopping power from the rear.
THIS IS THE ONLY THING YOU CAN DO TO YOUR BRAKES TO MAKE YOUR CAR STOP FASTER. Bigger brakes only give you the capability of stopping longer before brake fade.... and the hydroboost only gives the peddle a better more linear less spongy feeling.

hope that helped.. now back to building my new home...

you guys rock!!!


TRUMP

 

My brass prop valve/sw assy in my '72 had a strange habit, letting air into the lines....I finally got pissed and eliminated it.....rear master cyl port goes to rear brakes directly and forward most m/cyl port goes to front brake splitter, same as in rear brake line in back.....so no switch, and no air in lines either.....

I have an aftermarket aluminum m/cyl pirate Jack racing, and it's stock size....and also Hydroboost brakes..... car stops fine, but I have very strange wear pattern problems in the front only, with the O ring pistons....

 

I have a '69 Coupe which I am less-than-impressed with my braking performance.
It's a manual brake car, which I haven't driven in many years, so I've been assuming that I just am not used to the lack of boost from a power system.
However, now reading these threads, I'm wondering if I have something amiss in the proportioning/distributing valve block.
The prior owner had snipped the wire at the valve block (assuming to eliminate the nagging light he was seeing). Not sure if that was telling him there was an internal problem in the block or not. I don't get any fluid leaks anywhere.
So, could it be, there was originally a leak issue that tripped the valve one way or the other to light the light. Then the leak issue was resolved, but the light stayed on. So Bubba just gave up and snipped the wire?
And this could be resulting in reduced pressure still going to either front or back brakes - wherever the original low pressure problem had surfaced???
If the valve is still in its "protect" position from a prior leak, even after the lead was fixed, could that be contributing to what I perceive as reduced braking performance?
BTW - Replaced or checked all pads last fall, and replaced front calipers (one had been leaking slightly at 1 piston lip seal).
Just wondering.
The car WILL stop, just not without a lot of force on the pedal. And it's not a spongy pedal that I'd associate with air in the lines.
Not the sports car performance I was hoping for, even out of a manual.
Also - where does the other end of the warning light wire go to? It's snipped a few inches from the valve block, and I know it somehow ends up connected to the BRAKE warning light in the dash (which does work when the park brake is applied BTW). Just not sure how it gets there.

 

It's kind of hypothetical at this point, isn't it? Probably you don't have enough time on the pads to see whether the rears are wearing at all. In a stock system they probably wear at half the rate as the fronts. The manual brakes are excellent for their time and pretty good still today. But if the fronts are doing all the work you'll notice the difference.

My suggestion is to first measure the pressure. I bought some gauges but you may be able to rent or borrow some. That will tell you definitively if the proportioning valve has gone hardover. They're easy to use, just screw them into the bleeders being careful not to let air into the system.

I think the stock master cylinder has completely separate chambers fore and aft and if you are only replacing fluid in the front that suggests the rears aren't wearing.

Be advised that the pistons fore and aft have different areas which ensures that the fronts exert proportionally more force.

As to that wire, I don't know where it goes but the switch probably shorts to ground which you can measure with an ohmmeter to see if it has permanently closed.

As to how much you want to redesign the plumbing that is up to you (e.g. proportioning valves). I wouldn't touch that until you've got the system correctly diagnosed and are satisfied or not with the performance.