seeking info: Dual MC conversion

[Trip]

This thread is meant to continue some research I was doing about a year ago and dropped the ball on. This could be very important to MANY Vair' owners so I'd like to make sure I complete the task this time around.


Last year I purchased my first EM car. I had heard stories of failed rear wheel bearings resulting in a complete brake system failure (as well as loss of parking brake and engine drag!!!) on early models, so for the first time I saw fit to invest in a dual MC conversion for one of my cars.

Shortly after installing the conversion I blew a front brake line during a test drive. I was very surprised to find myself with little or no braking action despite having a dual MC conversion installed!!! Only one line (on the front) had ruptured, but as soon as the fluid was depleted from the front half of the system I lost rear brakes as well which SHOULD NO HAPPEN with a dual MC setup.

Isn't this just the reason we do Dual MC upgrades?

My inquiries resulted in the following data:
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Cars with Dual MC conversions from our (much loved!!!) vendors seem to often have little or no braking action at all when fluid is depleted in the front half of the system, the result is often the same as if it were a single MC setup.

Cars with Dual MC conversions from our (no hard feelings!!!) vendors often have less braking action than expected and in some cases no braking action when the rear half of the system runs out of fluid.

Cars with original Dual MC setups perform as expected and retain braking capability in one half of the system when the other half fails.

Cars with "home grown" dual MC conversions often perform as expected depending on the setup.
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Further investigation has lead me to believe that the reason for this is the fact that the "conversion kits" we are offered do not include or make use of a "shuttle valve" system. If I understand correctly the shuttle valve system closes off a failed portion of the system and generally prevents the Master Cylinder from running out of fluid. I am told the factory Dual MC cars DO have this "shuttle valve" system, as do some home grown conversions that use a more complete setup.

The reason seems to be that if one half of the MC becomes completely empty the system may not have the travel to produce sufficient pressure in the remaining chamber.


In conclusion, it seems very likely that many of those who invested in a Dual-MC upgrade for their Corvair may not actually have gained the extra safety the system is supposed to provide. Our systems very likely need to be further upgraded with a shuttle valve system similar to that found in the original 1969 Corvair braking system in order to complete the conversion.

I do not put any blame on the Corvair vendor's who supply us this system, though I fear for whomever originally came up with the conversion kit I (and many others) purchased through them. This sounds like just the thing that could lead to one of those huge lawsuits we all hear about, and I certainly don't want any of our vendors caught up in anything like that!!!

If possible lets please quickly gather and consolidate data. Anyone who has one of these conversions on their car and wishes to help out can do some tests by opening a bleeder on one corner of the car and informing us of the results. I know on my car (with the conversion) my pedal will go right to the floor with any one of the four bleeders open. It would be best to test repeatedly until no fluid is left in one half of the MC as the situation may change (for the worse) once that happens.

Anyone who is very knowledgeable about brake systems (in particular with this shuttle valve stuff) please don't hesitate to chime in.

Once we have sufficient data I'd like to first let our vendors know what our findings were, and then do our best to inform the Corvair community if there is anything to be concerned about.

Thank you all!

[UNSAFE]

Further investigation has lead me to believe that the reason for this is the fact that the "conversion kits" we are offered do not include or make use of a "shuttle valve" system. If I understand correctly the shuttle valve system closes off a failed portion of the system and generally prevents the Master Cylinder from running out of fluid. I am told the factory Dual MC cars DO have this "shuttle valve" system, as do some home grown conversions that use a more complete setup.

The late model corvairs with the dual cylinder did have the valve that you are talking about. Many people think it's just for the brake failure light but it does indeed close off the failed circuit and directs the fluid pressure to the other circuit. .

[joelsplace]

I would want to study the design of the master cylinder in question. I had one on another car give me fits until I really studied it and figured out that it was designed wrong and would never uncover the return hole when you were not on the brakes. It caused the brakes to drag severely when hot. (disks) You should be able to study the master cylinder in question to see why it failed that way. Was it really still working but the pedal travel increased so much that it hit the floor before the brakes worked? Does it rely on pressure on one side of the plunger to make the other one work? All the duals I've had apart applied pressure to the front plunger via fluid pressure from the rear. If there was no fluid in the rear then the pedal traveled a greater distance and the rear plunger contacted the front.
All of the moving brake failures I've had over the years were bad master cylinders and the other half still worked but the pedal went almost to the floor.

[Trip]

Was it really still working but the pedal travel increased so much that it hit the floor before the brakes worked? Does it rely on pressure on one side of the plunger to make the other one work? All the duals I've had apart applied pressure to the front plunger via fluid pressure from the rear. If there was no fluid in the rear then the pedal traveled a greater distance and the rear plunger contacted the front.

My belief is that the above is essentially correct. The catch is that if there was a working shuttle valve in place it "should" have closed off the failed part of the system and prevented the one half of the MC from going dry. If this had happened the MC would then still be able to apply sufficient pressure to the good half of the system before the pedal hit the floor.

[Trip]

I found this on a corvaircenter post:

The drawings I've seen show an "H" shaped piston in a connector tube that runs between the front and rear circuit.

High pressure differential (due to loss of fluid) causes the piston to shift and block the failed side. The "H" shape: When centered, the switch contact is not grounded because it's in the low part of the "H". When the piston moves off-center,the piston contacts and grounds the switch terminal.

As I recall, the driver has to stomp the pedal to engage the safety valve. (In my understanding, gentle braking won't do it; this is why we can bleed the brakes without activating the switch).

I have not re-centered a Corvair safety switche, but some of the early ones required some gymnastics. Stomping on the brake pedal sometimes did it, cracking a line on the good side and gently pressing the pedal sometimes did it. Some had a centering button. Anyone have any experience re-centering a Corvair safety switch?

Craig N. Coeur d'Alene ID.
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I do have at least one report of a factory 69' experiencing "near" total brake loss with the front line ruptured, but based on the above perhaps the fluid all leaked out of the front half before the pedal was "slammed" hard enough to make the valve work? Or perhaps the (at the time 10 year old) shuttle valve was stuck?

[Trip]

This part either needs to be included with the conversion kits, or the buyers of these kits need to be informed that the "dual" system is incomplete and is likely not to function as intended without it.

We have at least one report of a factory system failing to work despite the fact that it had the part, but if the "complete" system fails it is on GM, if someone wrecks a car or is hurt/killed with a Dual-MC conversion that did not include this part the blame might fall on whoever designed or provided the conversion.

[TexasUK]

Doing a search before I buy one of these. Was there an update to this, as in is the system still worth it?

[Bobby65]

i took the master cyl from a 67 along with the shuttle valve to replace the system on my 65, and i blew a rear hose and the pedal went down alot farther than normal but i could still stop, i have always wondered why no kits include this part?

[TexasUK]

Anyone know where I can get a shuttle valve please?

[UNSAFE]

Anyone know where I can get a shuttle valve please?

I think what is being called a shuttle valve is more commonly called a combination valve.

These are available aftermarket from places like MP Brakes, or I think most GM cars with drum brakes from the same
era likely have the same valve.

http://www.mpbrakes.com/products/produc ... uct_id=614

The quoted post from Craig sounded kinda odd because when I said a few years ago that one purpose of the
valve was to close off the faulty circuit he said that he took one apart and it didn't have any shuttle device .
I guess that's what's called a flip-flop

I believe that the valve also porportions the pressure to allow the rear brakes to react first to prevent nose diving.

On my 4 wheel disk brakes I could not get a pedal that felt good until I installed a combination valve .

Here's a pic of mine .