How can you tell if you have a bad brake proportioning valve? :crazy:

 

Some of these brake shops seem to always say you have a bad proportioning valve and quote you a grand when probably all you need are new pads and rotors turned.

One good clue to a bad proportioning valve is if it pulls very hard to one side when you step firmly on the brakes. My IROC does that, and I'm sure I need a proportioning valve.

 

Thanks much,Bearcat

 

what if no matter how good or how many times you bleed your brakes, it stops mostly on the fronts?
does that indicate a bad valve too?

 

Corvettes ( at least most older ones) DO NOT HAVE PROPORTIONING VALVES. Sorry to shout, but this is one myth that I wish would just die.
What they have is a distribution block with a switch in it to warn of pressure loss on one side of the system.

There's nothing really that can go wrong with them, and I 've never heard of a need to change one.

Why do you ask?

 

Guys you know my opinion of proportioning valves. I don't run them. I find with the valve bleeding the back brakes is a hassel and once removed they bleed easily and the car stops on all 4 wheels. Next to the hydraboost it was the best thing I did for braking.
Ditch it.

 

a pull to one side generally means a bad caliper. AFTER you make sure it isnt the caliper then the proportioning valve is next to look at.

On a related matter, i've never had to replace a proportioning valve. I had 2 that were acting funny, and i just dropped them in some solvent overnight, sprayed them off with brake cleaner and compressed air then reinstalled, never had a problem with them again.

 

Mike,

Right on the money... In fact, GM used to list it as a "SWITCH" in their parts catalog when it was still available. The only 1968-1982 Corvettes that I'm aware of that ever used a true proportioning valve was in a few early build 1968 L-88s that were equipped with the J-56 brake system... Later in production it was eliminated as part of the J-56 option.

Also true which is why it would be one of the last places that I would look for brake problems.

Regards,

 

Mine started to leak and I lost my rear brakes. Just bought a new one for $75. I was surprised anything happened to mine. They are usually the only brake component that never needs to be replaced.
~Christian

 

i find this VERY interesting...
is there anybody else here that has removed theirs?
and, if so, what was the result?

 

Sign, I am fixing to do just that maybe sometime soon, I allready have the lines and parts/adaptors I need, thing is I STILL can't prove the thing/switch is actually bad, even though it looks like it....seems to weep DOT5 out the switch itself....I just do't think the pedal is where it needs to be, and I swear I can't beleive it's the stock booster yet....and yet it still might be the silly rear calipers....had my fill of brake problems....O ringed them...problems go away for few months, come back...change master cylinder out, put in truck cylinder, change out lines, leaking rear long line under driver's frame rail....
someday I will win....

GENE

 

Later C3s did use a proportioning valve. I believe the first year for use across the board was '77. Brake bias with four-wheel disc Corvettes prior to '77 (and can't say regarding the J56 option) was accomplished through the difference in caliper piston area. Anyone who has pulled the caliper will notice the the rear caliper pistons are significantly smaller than the fronts though the housings themselves appear similar.

The brass block that many refer to as a proportioning valve in most C3s, and has already been passionatly pointed out, is just a distribution block with a pressure differential warning switch. It has nothing at all to do with brake bias. And even in C3s that do use a proportining valve, the bias is always rear and front. It is never side to side so if you have a pull, it can't be in the valve. There are some cars, Volvos for instance, that use a cross circuit hydraulic system where if there is a failure in one line, you retain braking in at least one rear and the diagonal front. It's pretty complex but Volvo builds their reputation on safety and that design type is line with that engineering philosophy.

Brake bleeding for the rear brakes without a proportioning valve should be easier since you aren't dealing with the movement and delay of the valve. Though bleeding with a valve isn't significantly harder.

 

Not true! As gerry72 has said, the DISTRIBUTION block "monitors" front to back pressure and engages the switch if that distribution changes.

I had to replace mine, and the indication was that I could NOT bleed the rear brakes after replacing EVERYTHING but the distribution block!

 

On a car where the rear brakes fail, you could expect the rear end to try to come around in hard stops & if the fronts fail then poor braking.

Since C3 disc brakes are sized differently ft. & rr. & wt. balance is 50/50 (or near) there is less need for a proportioning valve. No problems w/ FE-7.

:cool:

 

This subject seems to get beaten to death but no on tries it except me. I have my front brakes plumbed to the front master and the back to the rear, No connection between the two, no switch/proportioning valve, nothing and you can open a bleeder screw in the back and the pedal will feel normal in the front. The car still stops hard with no pedal loss. I have not tried it with the front open.
My car immediately felt alot stronger once the valve/switch was removed, the car seemed as if someone grabbed the back bumper and was hanging on. With my stock 75 non power brakes it felt like a huge improvement. Also from a safety point the pebal would no longer go to the floor on the first panic stop, no pumping up.
I went ahead and did this same mod on my 89 mustang that I was having problems with. And guess what. A huge improvement of it too, back bleeding went so much easier, and I was having a problem.
In my wifes car or my work truck, NO. But I drive in good weather only, usually have aggressive tires on the back, so why shouldn't they do more work and NEVER ever drive in the snow.


[Modified by norvalwilhelm, 10:57 AM 7/10/2003]

 

Norval, you know I respect your opinions on here pretty good, and since I have got the material to do the damn job...misery of miseries, I guess I will drag my carcass under there for the thing....might a well, one more thing eliminated...and as you know, i'm in Florida...so 'never' see snow/ice...
well really now, neighbors tell me not since Christmas of '89 anyhoooo....

GENE

 

I did a bunch of experimenting with the switch on my 70 and with the switch on an 80 a short while back... should be in the archives. I did these tests as a direct result, and for the specific benefit of people here on this forum. I have not yet published the tests anywhere else.

I removed & disassembled & photographed the inside of the 1970 and the 1980 switch. The tests proved that some guys on here were right, and some were misinformed. The actual performance of the switch "in-service" cannot be argued with..... what you see is what you get. That's why I did the tests. "The BS stops when the green flag drops." Unfortunately, the 80 was sold before I could conclude the tests on that car. But the 70 tests were intact.

Besides leaking to the outside, the switches also fail internally. Mine had failed internally... it was frozen in the "normal" position. There are o-rings inside, and they deteriorate over time. This, in combination with sludge, causes them to hang up.... regardless of how much pressure the master cylinder generates.

If they hang up in the normal/center position, then you will be able to bleed your brakes easily. If they hang up in the "error" mode (off center)... you will not be able to bleed either the rear or the front. If the valve/switch is working properly, it is difficult to bleed the brakes because each time you pressurize the line, the switch slides to one side, cutting off flow/pressue to one-half of the braking system (and it generally stays there until manually reset). Remember... I tested all of this on an actual car... no books or theory here.

One thing that I did note, is that general comments about C3 "proportioning valves" cannot be made.. there were several designs over the years, and they had different internal configurations, switches, & valving(re-routing of fluid). Each valve/switch must be discussed relative to itself.

If you are having a problem with a 76.... then you need to ask questions specifically for a 76 switch.

The switch isolates front & rear circuits... there is no side to side isolation in a C3 switch.
As posted... if the car pulls to one side, it's not likely to be the "proportioning valve".

As far as removing the switch goes.... the tests I did sort of answer that question.

In favor of removal... well discussed by Norval.
I have a 1966 Vette without power brakes, a single reservoir, single line system. I drive it without fear, since I maintain the brakes on it myself. This is primative, but basically equivalent to Norvals setup.

Not In Favor... SUPPOSEDLY (see tests if you can find 'em), the switch will save your life if you lose all pressure in either the front or the rear, as originally intended by GM. To test this, I first rebuilt my entire 1970 brake system and bled it so that it was functioning as it did the day it rolled off the assembly line. Then, I disconnected the front output port of my master cylilnder and installed a loop back to the reservoir. There was total pressure loss in the front circuit, and the valve (switch) was operational. Then I test drove the car... it would have slowed me down (maybe) but I don't think it would have saved my life.

Hope this helps. If not, search the archives. :)

 

Gene this is not a misery job. You should be happy to have something to do on the vet. I wish I could find something to do tonight. I have nothing that needs doing right now. It sucks.
I have got my bump steer down to .015 on the down but still 3/8th toe in on 3 inches up. That needs addressing.
Gene eliminating the switch is a good move. It totally seperates the front and back so one doesn't interfer with the other. I lost rear brakes once and almost never got the truck stopped. I pumped like crazy. It would take 2 or 3 pumps every stop. Remove the valve and on more pumping if one end goes out.
Just remember the crazy Canuck said to do it. :) :)

 

i sort of tried this on my 67 galaxie.
this was in 1990, so i was not as well informed.
Anyway,
i ran the rear direct to the MC.
But(mistake) i left the front connected to the valve and plugged the rear outlet.
So now if one end blows out i have good brakes. but if the opposite end goes it has no brakes. :lol: :eek:

 

Great info guys, this is why i love this place. I'm doing a complete rebuild of my 77. After reading what NORVAL wrote, i'm not inclined to re-install my switch, but to plumb it dirictly to the MC.