Brake warning light on

You were correct to check the fluid level. The reservoir has one of three switches that will turn on the warning light. The second most common is the parking brake. When the light turns on again, pull the pedal up with your toes, or push the handle down for vehicles with the pull-handle between the seats. If the light goes off, the main parking brake cable is stretched and must be adjusted, (more common if you use the parking brake a lot), or one of the rear cables is sticking and not fully retracting under normal return spring pressure. This is more common when the parking brake isn't used very often.

Caradiodoc

Your description is dandy. The only other switch is part of the combination valve near the master cylinder. It turns on when there is a leak in one half of the hydraulic system. Normally the light turns WHILE you're pressing the brake pedal, but if it tripped recently, it can do what you're describing.

Before you go searching for that switch, and assumming nothing changes when you play with the parking brake, unplug the switch from the master cylinder reservoir while the light is staying on. If the light goes out, if the fluid isn't low, check the styrofoam float to see if it's sinking.

Caradiodoc