Help a 3,800lb car to not hurt anyone at Summit...

Is there a pressure differential valve-- meaning is there a safety feature whereby when you lose pressure to one half of the system it slides a shuttle valve over and closes the weak side off allowing you to still "safely" stop? Might you have triggered that?

Is your brake light on?

Late 60s Mustangs (dual chamber master cylinder) had a valve in the plumbing that seemed to look for pressure in both circuits and lit up a light if not.  A little brass thing with wires coming off it.  I'd look for one of those and check it.

Thanks guys.

In order of the Q's received:
Unfortunately the MC came with zero documentation, so I'm not sure if it has a diff valve.  Even the box had no writing on it.
Yes, it's a dual chamber MC.  The rear flex hoses are still in 'like new' condition.
The MC is literally the only thing left from the original brake system and I didn't reuse the idiot light thingy.

I'm not sure, but it sounds like the MC has a diff pressure valve.  How would one remove said valve... pull the piston assembly out of the MC?  Is it possible the valve is actually in the distribution block?

To avoid tripping that safety valve, you can also gravity bleed the brakes - but that takes a heck of a long time.. 
Another option is to build a pressure bleeder using a yard sprayer, like so:
http://faculty.ccp.edu/faculty/dreed/ca … /index.htm

To un-trip the safety valve, you need to close all the bleeders and press the brake pedal HARD... or like everyone else said, ditch the factory combination valve and go with a cheap jegs/summit/speedway adjustable proportioning valve.

-Jason

2 items that may or may not help:

1)  late 60's - 80 Triumph products have a shuttle inside the distribution block that does just what you've described.
2)  There's an electric switch that lights an idiot light on the dash if the shuttle is moved.  I have heard that a Ford switch is an exact replacement for the NLA Triumph switch.

If there's an unscrewable plastic switch in the center of your distribution block, unscrew said switch.  If it's like the Triumph block, you will see a small shuttle that you can recentralize with a sharp point (ice pick, dental pick, etc.).  It will make sense when you look at it...good luck!!!!

I figured it was in the distribution block and I do have the plastic bit/sender.  I'll unf**K it tomorrow and then look to source a distribution block w/o the "pressure differential valve".

Thanks to everyone here!

Just hit the JY and pull 2 tees from any similar (american) car. There is usually one in the front to split the front line, sometimes on the firewall. There is also one on the rear axle to split the line after the flexible line from the body to the axle.