<VV> Dual MC total failures: solution I think

[Ray "Grymm" Rodriguez]

I have spent my spare time the last few days researching Dual-MC systems, 
the way they work, and the ways that they fail.

After communicating with more knowledgeable folks, reading a number of 
online articles and tutorials, and examining many diagrams of Dual-MC's I 
believe I have a three part answer to the question many of us are asking:

Why have so many of us experienced a total loss (or very near total loss) of 
brakes when we had a failure in the front brakes when we thought we were 
protected by a Dual-master cylinder setup?



#1-  Rear brakes are out of adjustment- sometimes even just slightly will 
cause this scenario.  Here is a quote from one of the online articles:

"If there's a loss of pressure in the rear cylinder (the one controlling the 
front brakes) the rear piston comes into physical contact with the front 
piston (the one controlling the rear brakes) and the pedal is now 
mechanically connected to the rear brakes.  The problem with this scenario 
is that there isn't enough pedal travel before you hit the carpet to get 
adequate application of out-of-adjustment rear brakes."

The reason given for this failure being so common is that many folks driving 
habits (particularly conservative driving habits) do not adequately provide 
the neccesary motions to actuate the automatic adjusters on our (63 up in 
the case of Corvairs) cars.  Something else mentioned (though I dont know 
how true it is) is that you must release the brake pedal between the forward 
motion and rearward motion in order for the adjusters to be sure to do their 
thing.  Especially in the case of automatic cars this often never happens. 
In my case I had just installed new shoes and I guess they were not tight 
enough, I don't think they weren't very far out of adjustment at all and I 
counted on the automatic adjusters to do the rest.

#2-  If this failure occurs when the brakes ARE properly adjusted the likely 
problem is that the pedal simply does not have enough travel to actuate the 
secondary piston when there is no fluid between the primary and secondary. 
If the primary half of the system is out of fluid the primary piston is 
expected to travel considerably farther than normal (giving a low pedal) and 
physically contact the secondary piston, then forcing it forward.  If you 
run out of pedal travel before this happens (or before it travels far enough 
for somewhat effective brake application) you will experience the same 
failure.

I plan to research this over the next few days and determine whether or not 
there is a problem with 66' and earlier pedal/plunger assemblies not 
providing enough travel and report back, but I strongly suspect the problem 
we have experienced is caused by adjustment more often than too short a 
plunger or pedal travel.

#3-  This one is common knowledge to many of us, but not to others who are 
perhaps less mechanically or scientifically inclined:  Even if you are 
getting 100% braking force to the rear wheels the car is not going to stop 
very well.  The front brakes provide nearly all of the stopping power on a 
vehicle so don't expect your rear brakes to stop the car reasonably well 
alone.  In the event of a front brake failure a dual MC system that is 
working properly will provide just barely better braking action than the 
parking brake handle.

In case anyone is wondering why this is true I'll give a brief explanation. 
Applying a stopping force to a moving vehicle transfers weight from the rear 
of the vehicle to the front.  Think of a motorcycle hitting its brakes so 
hard that it stands up on its front wheel. I'd also bet you've noticed (in 
movies at least) the nose of a car diving under hard braking and the rear 
end sticking up in the air. The greater the force of the deceleration the 
more weight is transferred forward.  The moment that you hit the brakes 
weight shifts to the front wheels and off the rear wheels.  This drastically 
increases the traction of the front wheels as they now have far more 
pressure (pounds per square inch of contact area) pressing them down against 
the road.  A theoretically equal amount of traction is LOST at the rear 
wheels where there is now considerably less pressure providing them 
traction.  The harder you brake the bigger this problem becomes and it 
doesn't take much at all under these circumstances for the rear wheels to 
simply lock up and start sliding, providing even less braking effort.  Like 
any brake lock up situation the key is to instantly back off the brakes 
until they stop skidding.  The maximum braking effort is achieved with the 
pedal pressed to JUST barely before the wheels start to skid.  When all four 
brakes are working the system is designed to take advantage of this 
situation by applying considerably more stopping force to the front wheels 
than the rear, but in the event of a front brake failure the rears are all 
you've got and they are not going to do a very good job.

Ray "Grymm" Rodriguez III
Lake Ariel, PA
CORSA member
66' Corsa 140/4 coupe
65' Corsa 140/4 coupe (under construction and almost done!)
63' Monza PG coupe (almost done!)