<VV> Dual Master cylinder upgrade.

[Ken Pepke]

> From: "Dennis Pleau" <dpleau at wavecable.com>
> Date: January 5, 2011 10:00:47 PM EST
> To: <fastvair at yahoogroups.com>, "'Virtual Vairs'" <VirtualVairs at corvair.org>
> Subject: Re: <VV> [fastvair] FW: Dual Master cylinder upgrade.
> 
> For your peace of mind if you have a dual master cylinder.  Have someone press down on the brake pedal as hard as they can and open a front bleeder valve quickly (simulating a blown line or hose).  Does the pedal go to the floor?  If it does, your dual master is not working as advertised.  If it works, try again on a rear wheel to make sure it works on both ends.  It’s an easy 10 to 30 minute test to let you know if your setup works and may save your life.
> 
Sure, the test is easy and also positively answers the question ... But, first take a look at the MC itself.  Does it have the curly 'pig tail' lines AND a rectangular block that looks like it is floating on the brake lines parallel to the MC.  I am thinking there have been home brew conversions that the builder, thinking they did not need the warning light, or, not even knowing about the block, did not install one.  After all, if someone just went to his FLAPS and bought the dual MC and a hand full of brake lines ... NO block!
Ken P
> 
> 
> I worked as a mechanic in the 70s and ran across the block that has been referred to on this list as a proportioning valve or a switch (it doesn’t proportion but does switch on the dash light).  Some and I don’t remember if they were GM shut off the circuit with the blown line.  Since the pistons in most dual master cylinders are hydraulically coupled, if you blow one circuit and don’t shut it off, you lose the hydraulic coupling and therefore both front and rear brakes.  
> 
Back in the day all the car companies advertised the dual MC as keeping the brakes working on one end if the other end failed.  There is really no other reason to have a dual MC.  I cannot remember seeing a Corvair commercial making that claim but by 1967 Chevy was not doing a lot of Corvair advertising.  
Ken P   
> 
> When doing the high pressure bleeding as outlined above, and you have a block that shuts off the flow, you can continue to try to bleed the brakes out of that bleeder and you won’t get any more fluid.  You will also not have any brakes on that circuit.
> 
> 
> 
> If you have the block is a shutoff valve on your car and you do this test, you will need to reset the block.  It is done by having your partner press down on the brake pedal with a lot less pressure and slowly open a bleeder on the circuit that is still flowing fluid until the brake warning light just goes out and close the bleeder.  If you don’t have a brake warning light, you will need to connect an OHM meter between the block and the light connection and close the bleeder when the resistance goes infinite.  
> 
Resetting that piston can be a trick.  I have worked for long periods making the attempt.  Perhaps someone knows a trick that does not rely upon dumb luck to hit the spot.
Ken P
> 
> 
> From what I’ve read in the last couple of weeks, I think there may be a Corvair owners who have dual systems that probably are not any better than the single master cylinders the replaced.
> 
> dp
> 
> 

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