You need to check to see if you have a check valve(residual pressure valve) between the master cylinder and the line that leads to the back brakes. This valve is needed on drum brakes to hold enough pressure in the wheel cylinder so the shoes expand out as soon as you push on the pedal, if it was not there you would experience a lag time where the brakes didn't work until the wheel cylinder filled with fluid.
One the single circuit/dual circuit master cylinder...At least in the U.S. all cars after 1966 came with dual circuit brake systems to prevent a complete system failure if a line or wheel cylinder fails. So anything that is 66 or older (my Ghia just had this problem) will have a single circuit set up. VW made two different dual circuit master cylinders, the ones that had the residual pressure valves installed before the brake line, and ones that had the residual pressure valve built in to the master cylinder(these would be late model units) I know of no way to change the late model unit to allow for rear discs, so if you have that one you will most likely have to change the master cylinder. Olspeed
66 Ghia, "Dexter" 76 flat window Beetle
It's not a Car it's a VolksWagon!