<VV> 69 brake system diagram?

[Dennis Pleau]

Ok, the light just went on!!!

Some of you know I was a mechanic in the early 70's and had a brake
certification from Union 76 after I took a class from the Autolite technical
institute (they still belonged to Ford at that time) and passed a test.  I
was trained and worked on a lot of the early dual master cylinder systems.

What turned on the light were the posts by Lon about the shuttle (switch)
and the dual master cylinder diagrams linked by Clark.  Clark posted a
diagram of a disk brake dual master cylinder, but they are almost the same
as a drum system, except the drum cylinder doesn't have the residual
pressure check valve.

First you need to look at the normally released diagram posted by Clark.
Note: the primary reservoir feeds the brake line (usually to the front
brakes) and the floating piston for the secondary system. 

Now look at the rear line ruptured diagram.  When the rear line is ruptured,
you no longer had a fluid coupling between the primary piston and the
secondary piston, but a mechanical coupling which takes a lot more pedal
travel.  The pedal is probably hitting the floor before the mechanical
coupling can put sufficient hydraulic pressure on the rear brakes to stop or
even slow down the car.  

This is where Lon's post comes in.  He described the block as a shuttle with
a switch placed between the primary and secondary which trips and
illuminates a light when there is a pressure difference between the primary
and secondary brake system.  He is right but safety wise, it is a lot more!
It is actually a shuttle VALVE with an electrical switch.  

When one system loses pressure, the pressure from the side that didn't lose
pressure pushes the shuttle valve from the high pressure system to the low
pressure system and the valve closes off the outlet of the valve on the low
pressure side.  In the case of blowing a line on the primary side, the
shuttle valve closes the line off 6" from the master cylinder hopefully
before the blown line or cylinder.  When the shuttle valve closes off the
blown primary circuit, there should still be enough fluid in the primary
side of the master cylinder to maintain the fluid coupling in the master
cylinder.  

I hope this makes sense to someone other than me.

I if asked will tell my favorite brake repair story.  It had to do with a
mid 60's chevy truck pulling a horse trailer with a horse, operating brakes
on three wheels and coffee for brake fluid.....

dp