Schematic for a proportioning valve

The block is labelled correctly going out to the wheels, its incorrect as far as which line feeds which side.

A master cylinder piston is actually 2 sizes in one. The smaller part of the piston is at the front. The larger piston is at the rear. The larger part of the piston assembly ports more fluid for front disc brakes.

Typically The front port of the master feeds the rear brakes part of the brake block. Rear port of the master feeds the front brake part of the brake block. However even though that's a majority of them, it's a general rule but not always the case.

Easiest way to tell which is which is look at the reservoir on your master cylinder. The larger of the 2 chambers is your front brake side, smaller of the 2 is rear brakes.