The master cylinder is the heart of the brake hydraulic system, and when it fails the symptoms are distinctive. Recognizing them tells you the master - not the pads or a leak - is the problem. Here are the symptoms of a bad master cylinder.
A slowly sinking pedal
The classic sign is a brake pedal that slowly sinks to the floor while you hold steady pressure - at a light, for example - with no external leak; see why the pedal sinks and pedal sinks in traffic. The internal seals are bypassing.
No leak but low fluid or soft pedal
A failing master can lose pressure internally without dripping a drop, so you get a soft or inconsistent pedal while the reservoir stays full. That "no leak but bad pedal" combination strongly points to the master cylinder.
Uneven or fading braking
Because the master feeds two circuits, internal failure can cause uneven braking or a pedal that feels different from stop to stop. The brake warning light may also come on if the pressure is unbalanced.
What to do
Confirm there is no external leak, then replace the master cylinder - and bench bleed the new one first; see how to bench bleed. A quality reman master is a good value; see reman vs new. Keep the system healthy with quality pads and hardware.