[RichardJ]

>>can be confusing that's why "Primary and Secondary" shoes and "Rear and Front" are used in the manuals? The primary shoe always goes to the rear! That's the shoe with the long lining. Short lining shoes are secondary and go on the front,<<

Yes, the longer lining is on the rear, but it is called the Secondary shoe. Mind, this only applies to single wheel cylinders, mounted on the top and both shoes rotate together in the direction with the drum and are self energizing.

Reservoir size has nothing to do with the operation of the brakes. The rear piston in the Master Cylinder is called the Primary Piston. The front piston is the Secondary Piston. Either piston and outlet can be used for front or rear. As volksworld pointed out, the Combination Valve outlets are very important and can't be interchanged. The Proportioning end of the Combo Valve must go to the rear brakes. The other end of the Combo Valve has a Metering Valve and it must be connected to the front brakes.

By the time Disc Brake Pads wear down to nothing, most of the fluid held in reserve in the Reservoir will have been transferred to the back of the Calipers. If the Reservoir is too small, that circuit will run out of fluid before you find out the disc pads are completely worn out. By then you are pushing air into the lines.

Modern brake systems use a single Reservoir that in most cases is probably too small. They use a Fluid Level Sensor in the reservoir to turn on a dash Brake Warning Light. If everything works correctly the light will come on a few thousand miles before the Pad Screech Bars start dragging on the rotors.
I've seen a few aftermarket replacement pads for newer vehicles that didn't have the Screech Bars.