Brake lights only work sometimes?

When I press my brake partially to slow down, my brake lights don't work. When I press it fully they do. Anyone have any ideas what this could be?

I checked the master cylinder and there is plenty of brake fluid, and the brakes don't feel "soft" or anything. I wouldn't have even known it was going on if some guy didn't flag me down and tell me my brake lights didn't work.

Brake light switch at your brake pedal is malfunctioning. Depending on the year of your Expedition there was a recall on your issue. Check with your dealer.
Franco