PAULDANA...PM sent.

 

This shouldn't be a sticky because the author does not understand the principle of the dual master cylinder and the function of the switch, often called a proportioning valve. The switch does not shut off pressure to the low pressure circuit! The only function of the switch is to turn the brake light on when there is a failure on one circuit - front or rear. (The secondary function of the light is to provide a warning when the parking brake is on.)

There are many good brake bleeding threads on this forum, this is not one of them!

 

Thx... got it, I will be looking into it this weekend, that is valuable info.. thank you!!

 

I have to agree with PeteZ06, but I won't get into the principles of the M/C for now.

Doing an inspection and finding 1 wheel cylinder leaking and relacing it and bleeding it before the M/C went dry is correct.

Under Recap: always do the M/C first because air is forced downstream. That is the point! You must bleed the caliper closest to the M/C first, on a Corvette that is the left front. If you do the right front first you are just wasting time and fluid.

The Brake Light Switch does not stop or cut off fluid from a leaking caliper, it is just a switch to tell you that there is a brake problem.

The trick to Recenter the switch will not work if you open one front bleeder and one rear bleeder.

If a front caliper caused the problem-than after replacing or repairing that caliper and getting a firm brake pedal- apply pedal pressure and open a rear bleeder-that will center the switch.
If a rear caliper leaked and was repaired than apply pedal pressure and open a front bleeder. again that will center the switch.

On the rear calipers, i agree bleed the inners first and then the outers.
PG.

 

The shuttle valve or pressure differential switch that operates the warning light is NOT a proportioning valve. They are absolutely two different functions. A proportioning valve is in essence a pressure bleed off valve that reduces pressure at a pre-set point to proportionately reduce rear line pressure as front brake line pressure continues to increase. This function reduces rear brake bias in hard deceleration as the car is transferring weight off the rear wheel to the front. The purpose is to prevent rear wheel lock up in panic stops. Adjustable proportioning valves allow the operator to change the set pressure point where the rear line pressure reduction begins.

 

*sigh*

Here's a diagram for you. Notice what will happen if that piston seats left or right. It will shut off flow from one port or the other of the MC giving you little to no flow for that circuit.

Read Tom454's post on the first page. He damn well DOES understand it.

 

Here's page 5-9 from the 74 Service Manual, do different model years have a different switch?
http://s243.photobucket.com/albums/f...rakeswitch.jpg
http://s243.photobucket.com/albums/f...rakeswitch.jpg

 

Yes... the switch differs based on year.

 

I knew there was going to be a lot BS when I posted my first response to this question. My response covered basic bleeding. The operation of the the "switch" I had also discussed in length in previous posts, somewhere in the archives. There was so much BS flying that I decided to run a test on my own C3. I opened up a caliper bleeder (left front) and ran a hose back into the master cylinder reservoir, then I test drove the car. It stopped, but just barely. I almost drove through my garage door. The "switch" on a 70 does in fact close off the port in an attempt to allow the driver to stop if one circuit (front or rear) suffers a major loss of fluid. But it doesn't work all that well. At best you'll get one or two shots at stopping. In my test, the fluid was re-routed back into the master cylinder via a hose. In the real world, the fluid will go onto the ground, and one half of the reservoir will run dry. This won't happen instantaneously though... it will take some pedal pumping to empty the reservoir.

On the centering issue... I disassembled my own 1970 "switch" and went through the exercises of testing. My reponse rergarding re-centering of the valve stands. That is what happened when I tested a properely functioning switch off a 70. I might lie, somebody else might lie, the books might lie, but the test tells the truth. Dry, compressed air worked the best for me.

I also put in a disclaimer that I tested only a handful of C3 switches. I did not test them all.

This topic has been beaten to death on this forum... that's why I ran the "tests"... to put an end to the debates. Obviously it didn't work. Oh well.

I would like to see somebody else take their car apart the next round of debates. This is getting old. Any volunteers?

 

I tested this debate by opening one bleeder and driving my 70. The 1970 switch does in fact close off one circuit, but not very well. I also took a slew of pictures, and made a lot of measurements on the switch, and diagrammed the relationship between the tiny pistons and the ports in the switch. I can do no more than that. It kinda irritates me that people will continue to render opinions on this even after somebody takes the time to actually take the system apart piece by piece, run tests, and provide the results (for free). The information I provided was based upon actually doing the work.

I have a suggestion... why don't you do it this time?
Prove me wrong.

 

You ain't kidding about the b.s., Tom. I think some guys just like to repeat what they hear.

Been there, done that

I don't have your brass ***** to drive around like that, Tom, but the first time I bled the brakes on my '76 I learned all about that valve. I cracked open a rear bleeder, wife pushed the brake pedal annnnnnnd ... nothing just barely a dribble coming out of the bleed screw. The pedal was hard as a rock meanwhile. Thus began my research, and I later learned that this valve had blocked off the rear circuit. Since then I've either used the gravity method or I've also found that if you use a very slow, gentle foot on the pedal when bleeding you won't slide the piston out of place.

I'm in the process of rebuilding my suspension, and I think I'm just going to plumb out that damned valve.

 

Those diagrams do not support your position.

 

I just went back and reviewed the pictures and my notes on the "switch" for a 1970. The switch consists of the housing, inverted flare inserts, the insulated off-ground switch post (& 2 o-rings), a central dog-bone, two springs (front/rear), two pistons (front/rear) and an o-ring on each piston.

Parts placement inside the housing from center, outward:

Dog-bone, springs, o-ringed pistons

The electrical part:
The dog bone is in the center, its travel limited when one end of the bone hits the protruding tip of the switch post. When the dog bone touches the switch post, it grounds the brake light circuit, through the housing, to turn the dash mounted brake light on. The dog-bone is made out of brass to facilitate a good contact. The switch post is steel, as is the housing.

The hydraulic part:
When a pressure differential is sensed, the circuit still with pressure (higher pressure) forces the o-ringed piston in that side to slide in the direction of the no/low pressure slide. This pushes on the spring, which pushes on the dog-bone, which pushes on the other spring, which pushes on the opposite piston.

Given enough pressure differential, the piston in the high(er) pressure side overcomes the springs and no/low pressure side hydraulic backpressure to force the no/low pressure piston as far in the bore as possible. When this happens, the path for fluid flow in the no/low side is restricted (not totally cut off) by the tip of the no/low side piston. In other words, the tip of the piston only partially blocks the fluid path through the housing. This is why it "almost" allows you to stop... but not very well. In addition, there are two springs in series with the pistons.. so this is not a 100% mechanical stack-up situation.

I very carefully measured the piston design, o-ring placement, and housing machining to determine exactly where the piston ends up in relation to the ports it's supposedly trying to block. I have the drawings somewhere in a folder, but never got a chance to enter them into a CAD system. I guess it's time I finished it up so I can post it all here.

Now remember...this is a 1970 switch. It's not representative of all C3's I also did this with 1980 switch.

 

Exactly... If you wrench on it yourself... you find out what works and what doesn't.

 

I think we have beaten this to death and time to close it else we get on each other's nerves....