Brake Master Cylinder Help

I would look for a master cylinder for a 67 or later drum brake A Body. You can hook it with a minimum of trouble. Use the brass junction block that's now on the car. Disconnect the rear brake line from the block & put a plug in that opening on the junction block. Plumb the rear brake line directly to the master. Easiest way I know of to do it.

If you want it to look more "factory original" there is a special junction block available, I think from Inline Tube, that takes the place of the original & lets you connect everything there. It's almost $40 though, & not really necessary.