66 Dodge Dart upgrade Dual conversion help!

Looks like there is a leak at that fitting? Usually when I see leaks at wheel cylinders, they are inside the drum around the pistons and dust boots. The repair necessary there could be as simple as a new run of 3/16 brake line from the brass block on the rear axle to the wheel cylinder. Or a line run and a new rear wheel cylinder. Both rear wheel cylinders on your car are same, so ordering one is not difficult. If you order front wheel cylinders keep in mind they are different for each side.
Please before you fix one little piece of this puzzle, you may want to investigate your entire situation. There are 6 pre-bent steel lines on your car from the factory, a single pot master cylinder, 3 hoses, 3 brake hose clips, a brass distribution block on the front frame rail under the master cylinder area, a brass distribution block on the top drivers side of the rear axle, and 4 wheel cylinders. If you want to piece in some fixes for these items, find which items you need. If you want much safer conditions replace everything and you know it is of good quality and safe.
You can order all the lines pre-bent in steel or stainless steel from Inline Tube. Check here 1966 Dodge Dart A Body Dual Master Conversion Complete Brake Line Kit Stainless | eBay. You need the new brass distribution block for the dual pot conversion here 1965-66 Mopar A B C Body Dual Master Conversion Distribution Block Tee Valve | eBay. Slantsix Dan recommended some great dual pot master cylinders. I used the Raybestos one. Worked great. As a matter of fact, I ordered everything else that I needed from Rockauto.com. They carry that master cylinder and all of the hoses, clips, wheel cylinders, brake shoes, hardware kits, and even the tools. You will need a service manual or you need to take pictures of the existing brake hardware set up. Make sure you get it put back together the way it came apart and use line wrenches on all the brake lines or you will round off those fittings. Good luck.
Doug