<VV> [fastvair] FW: Dual Master cylinder upgrade.

[Dennis Pleau]

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.

 

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.  

 

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.  

 

>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