First, you should have a Service Manual and a Parts Manual.
The reservoir is beneath the panel at the top center of the firewall with a T fitting at the bottom with a brake line going to the left and right brake cylinders at the rudder pedals. The reservoir is filled by loosening the nut in the center of the lid or pumping fluid up from the wheels. There is also a vent line running to the right that terminates behind the right gear.
There are a number of places where the system may leak, including; O ring at the lid of the reservoir, T fitting at the bottom of the reservoir, brake lines at the T fitting or at the Cylinders, the cylinders themselves, the lines from the cylinders to the parking brake assembly, the parking brake assembly under the floorboards or the lines from the parking brake assembly to the wheel brake assembly.
ATF and 5606 are not the same.