Assuming it is a pressure differential switch, and not a proportioning valve, the switch should keep the front and rear fluid systems completely separate. Under failure conditions, the pressure differential switch ought to make no difference to the braking system, other than activate the light.

For the pedal to go to the floor, are you suggesting that the differential switch can operate or fail in such a way as to kill the good half of the braking system in the event that the other half fails?

Perhaps folks familiar with the inner workings of this switch might be able to comment on this. Personally, I'd be amazed if GM could get away with such an unsafe design without being sued to eternity!

In any case, I'll have to have a closer look at this question, just for peace of mind.

Regards,
Joe

 

I have mine off right now and it is no way a proportioning valve, not in any way I can see. It has two seperate chambers one connecting the front half of the master cylinder with the front hydraulics and the other connecting the rear of the master with the rear. Between the two is a normally open switch that is grounded when the pressure is unbalanced and pushes a piston of sorts one way or the other and grounds the switch and lights the dash light.
OK, we all knew how it worked but I wanted to say it anyway :rolleyes:
As far as I can tell the two sides of the system are completely isolated inside the valve. Seperated by the piston which moves back and forth but passes no fluid.
So how does it allow total brake failure when one side is open :confused:

 

People get confused about C3 brakes because we assume since they all have 4 piston calipers, they are all alike; one of the chief differences is whether or not your car has a combination valve or a proportioning valve. Here is a quote from the Haynes manual:

"Two basic variations of this valve have been used. On 1968 through 1974 models, the valve includes a pressure failure warning switch which illuminates a warning light if hydraulic pressure drops in either the front or rear brake circuit. On 1975 and later models, the valve also includes a porportioning function. Under heavy braking, the proportioning section reduces hydraulic pressure to the rear brakes to prevent early rear wheel lock-up. The valve is also designed to assure full pressure to one brake system should the other system fail. The design of the switch and valve are such that the switch will stay in the 'warning' position once a failure has occurred. The only way to turn the light off is to repair the cause of the failure and apply very high brake pedal pressure."

Also, I know it is conventional wisdom on this forum that brakes should be bled from the caliper farthest from the master cylinder to the nearest. This is usually postulated in threads where all the participants bemoan the difficulty of getting all the air out of the lines, or of achieving the much desired "firm pedal." This is a quote from an original 1973 Chevrolet Service Manual, copyright 1972 General Motors Corporation. Section 5-18 in the Brakes chapter.

"It is advisable to bleed one valve at a time to avoid allowing fluid level in reservoir to become dangerously low. The correct sequence is to bleed the valve, either front or rear system, nearest master cylinder first. This sequence expels air from lines and calipers or wheel cylinders nearest to the master cylinder first and eliminates the possibility that air in a line close to the master cylinder may enter a line farther away after it has been bled."

Now that is mere common sense. When you press the brake pedal to bleed the brakes, the fluid obviously moves toward the wheels, not toward the master cylinder. It also makes no difference whether you bleed the back or front wheels first. Note the two reservoirs of fluid in the master cylinder; they are separate and do not exchange fluid or air so long as the master cylinder is functioning properly.

Now, here's the kicker I don't know the answer to, since my 1973 Corvette has a combination, not a proportioning, valve. Many of you guys who have 1975 and later cars WITH proportioning valves should check into whether it needs to be mechanically held open during brake bleeding. During 1973, some other Chevy cars, not the Corvette, used proportioning valves. If you used pressure bleeding, the following paragraph applied to those cars:

"NOTE: On disc brake cars a combination metering, proportioning and failure warning switch is in the hydraulic system and mounted either on the frame rail or under the master cylinder. This valve must be held in the open position while bleeding. This can be accomplished by installing Tool J-23709 with the open slot undr the mounting bolt and pushing in on the pin in the end of the valve (fig. 38). Be sure to re-torque the mounting bolt after removing ToolJ-23709."

I don't know if later Corvettes used the same type of valve as mentioned above, but if you have trouble bleeding your brakes, I would check the manual.

After messing with these cars for a long time, I've found that Chiltons and Haynes manuals are too generic to be of great value. It is worth the money to buy and utilize the Service Manuals for your year Vette. I trust the people who built the car to know how it needs to be fixed; the service manual for your specific year car is the first place you should look.

Best regards,
Eddie

 

Thanks Eddie,

That was a very useful post. There is a lot of mythology in Vette brakes.

I was unaware of the differences between early C3's and later. Mine is a 69 and only has the pressure differential switch. I agree completely with the comments on bleeding sequences and have had no real problems bleeding brakes (one of the lucky ones!). It seems "Yellow 72" has a similar system.

My reason for posting here at all, is that I couldn't believe that in a (properly maintained) dual circuit braking system, a single failure in one half could lead to failure of the remaining half.

Thanks for your advice,
Joe

69 427L36 M21 3.08 White Coupe

 

possible reasons for the failure:
1. that little piston leaks bad when it slides over to work the switch, connecting the good side to the bad side.
2. the piston moves enough that it creates a cavity big enough to cause the good side to lose pressure
3. poor engineering allowing 1,2; not checked in the real world



[Modified by drives61, 7:01 AM 6/19/2002]

 

This is very easy to prove or disprove. Any time you have a wheel off bleed whatever wheel is off. Open the bleeder valve while someone pushes the pedal and if it goes to the floor you have lost your brakes. If the pedal goes 1/2 way the other system is still getting pressure. All you loose is a little brake fluid and you know if loosing a line will destroy the brakes.
Anyone who has bled their system knows what happens to the peddle while bleeding.

 

That is what happens when I open one side...no brakes. So the valve is either defective or a bad design.
I can't recall this on any other brakes I've bled :cheers:

 

Well, no, not exactly. If you have the pre 1975 brake distribution switch (non proportional), I'd say you had problems. But if you have the proportioning valve, the test you propose is like buckling one of the inertia reel seat belts and moving the shoulder belt back and forth and determining that it is defective because it slips. So obviously, it wouldn't work in a crash? The two errors people are making by saying the valve must be faulty if you crack a bleeder screw and the pedal goes to the floor are (1) the car is at rest, unless you are agile enough to crack a bleeder at speed, and (2) you assume the proportioning valve operates at normal brake pedal pressures. Neither is true. The proportioning valve improves "front-to-rear brake balance at high deceleration. During high deceleration stops, a percentage of the rear weight is transferred to the front wheels."

But, more importantly, "The proportioner does not operate during normal brake stops. Fluid normally flows into the proportioner, through the space between the piston center hole and valve stem, through the stop plate and out to the rear brakes. The spring loads the piston so that it rests against the stop plate for normal brake pressures."

The above quoted material is from the 1973 Chevrolet Service Manual, copyright 1972 General Motors Corporation. Anyway, with the symptoms some of you have mentioned, I'd check the mounting bolts for the master cylinder and / or an incorrect master cylinder push rod adjustment, or just a garden variety defective or non-bench bled master cylinder.

Most important of all, before we remove or modify an integral part of the most important safety system of our precious cars, we should ask ourselves, as Richard Newton did in much of his writing, "Am I really smarter than Zora Duntov?"

On top of that, I can't believe this thread was started about a Mustang.

Eddie

 

Depends on the subject.

Pretty much uncalled for.

 

someone should call/write NHTSA and find out exactly how the mandated dual brake system should function.
my experience is with older cars, but i never saw one that had decent brakes with, say, a cut rubber brake hose. Seems the dual system is, at least in old cars, a total failure.

 

AHHHHHHHHH HE SAID MUSTANG!!!

:)

 

Mustang! Mustang! Mustang! Aaah Haa Haa Haa, watch 'em squirm....

Hans

 

Norval is correct, the pedal does drop much further if you open one of the bleeder screws and we have all experienced that.

So, let's assume that the differential pressure switch does keep the front and rear circuits separate. Now if we apply the brakes and open a bleeeder screw the pedal drops. See http://www.howstuffworks.com/master-brake1.htm
for an animation of what happens. You can click on the red arrow to see why the pedal drops.

However, the system is designed to reach full pressure on the remaining circuit. This SHOULD happen before the master cylinder bottoms out or the pedal reaches the floor, even though the pedal travels much further.

I guess if it doesn't, that means there are other problems with air in the system or master cylinder problems.

Check that link out and let me know what you think.

Thanks
Joe

69 427L36 M21 3.08 White Coupe

 

Great thread.

Although I am surprised nobody's chimed in to say that removing the proportioning valve will increase global warming, kill all the whales and turn innocent children into **** stars.

:lol:

 

Neat site. I think the system shown, with the electrical fluid level sensor, is a little more modern than our antiques, but the principle is the same. Perhaps if a failure happens during some of our more, shall we say "spirited" driving, 50% brakes feel like no brakes as we approach the rear of a parked police car at double the speed limit. At least we've got that wonderful parking brake right there to haul us down on a dime. :lol: <--- I don't usually like to use these little animated thingies, but for the accountants among us who don't understand sarcasm, I guess I must.

Eddie

 

"Parking brake :lol:" .....Iget it :lol: :lol: :lol:

 

The more I read this thread, the more confused I get! Take it off, leave it on, on, off...... :crazy:

I'm just afraid of replacing/changing one more thing that makes no difference to my spongy brakes!

 

I will make an assumption here.

Yes, when the bleeder is opened the peddle will go down, perhaps to the floor.

Now, in the real world, if a line blows, the peddle will also go down and the differential in pressure between front and rear will transfer the "insides" of the proportioning valve to what ever end has lost the pressure.

Now, on the second pump, you "should" have a relatively firm peddle, since the valve has now already did it's thing.

When bleeding, typically no one ever gives a second pump with the bleeder open, so we don't know what happens on a real failure.

 

barry
i used to give 6 to 8 pumps before closing the bleeder, depending on the capacity of the MC. they never "pump up"

of course now i use gravity on my 72.

 

Pretty cool information guys...

I wonder if ya could run the lines in the shape of an x... so left front is pared with right rear, right front to left rear.... so if ya lose one, it moves the load to the oposite corner :confused:

A friend of mine was talking about this, maybe it was on an 86 Celbrity? might have been the abs system or something... :crazy: