Installing a Dual Master Cylinder
In 1965, safety in cars was not always paramount. In 1967, the US government mandated that all cars were to have dual resevoir master cylinders. This provides better safety in the event that you have a leak on your brake system. On Single Resevoir Systems, if you have a leak on any of the brakes, all 4 brakes will be lost. If you have ever tried to stop a 65 mustang with the parking brake, you'll realize this is not a great solution. On a dual res. master cylinder, if you have a break in the line, you only lose the front or the rear brakes.

Parts Needed:

1) Dual MC for 1967 Mustang
2) Pre-flared Standard brake lines
3) T-Union (or buy a new dual res distribution block)
4) Reducer fitting for the rear brake resevoir connection

Tools Needed:
1) Line Bender
2) Socket Wrench

Optional:
1) Flare Kit

  

Generally, I would follow these instructions. I will provide some more pictures and some hints that may make it easier.

1) Remove Single Res Master Cylinder

As you can see, my old single Res "Fruit Jar" has some serious sediment in the system. Definitely time for a change.
2) Install Dual Resevoir Master Cylinder. Reattach the pushrod boot. For the 65 mustang, keep the original single Resevoir pushrod in place. The one that comes with the 67 MC is too long. You may need to remove the pushrod from the 67 MC.
Hint: Putting a weight or brick on the brake pedal will help extend the pushrod, making it easier to fit the boot onto the dual MC base.
3) Once the boot is in place, bolt the dual res. MC to the firewall.
4) Now it's time to Bleed the MC. It is not hard to bleed the MC when it's in the car. In fact, I think it is easier since it can be difficult to get the MC in with it filled with fluid. My MC that came from Autozone came with a bench bleeding kit. Install the fittings and tubes.
 
5) Fill the MC with fluid DOT 3 brake fluid.
6) Slowly pump the brakes to push fluid through the MC.
7) Keep an eye on the fluid level in the MC. Do not let it drop too low. Pump the pedal until you get a solid stream of brake fluid with no air bubbles in the tubes.
8) I capped the fittings for now until I was done with my lines. The easy part is done.

9) This is where you will start encountering some difficulties that no instruction set I've found prepared me for. Each situation is different, so be prepared, especially if the brake parts are original to the car (or have been on there a long time).

You will need to either get a new distribution block, or you can do like I first tried. I could not get the front brake lines free from the distribution block, but I was able to remove the rear brake line from it. I first plugged the rear port on the distribution block, and ran the rear brake line right to the front port of the MC. I routed from the rear port of the MC to the distribution block.

This worked fine...for awhile.
While My original set up worked fine, after taking a 5 minute test drive, one of my front brake lines parted in the engine bay. I was definitely thankful of my timing to get the dual res MC in place, or I would have ended up hitting my garage at 25 MPH.

Here is my final set up. I removed the distribution block all togther. I also ended up routing new brake lines as when I tried flaring the existing lines (or bending them) I found they were too old an fragile and were breaking apart.

I just used a simple T-union fitting to act as the distribution for the front brakes, and kept the rear brakes attached to the front port. This cleaned up my final brake line arrangement as well.
Now for more fun...time to bleed the brake lines. The easiest way to do this is with someone helping you buy pumping the brake pedal. I tried using a vacuum pump, but after an hour, the pump was broken and my brake lines still had air in them. 10 minutes with the help of my wife pumping the brakes on command, and the brakes were bled.  
 
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