The sensor looks for imbalance between the two circuits of the brake system. When that happens the piston you mention is forced to one side or the other by the presence of unequal pressure. The piston makes contact with the ground and the completed circuit lights the brake failure light.

You have reset the warning light by centering the piston. You did good.

A leaking reservoir alone will not let air into the system as long as you did not drive it long enough to cause that circuit to call for some brake fluid to make up for piston travel due to pad wear.

Don't worry about the light. How do the brakes feel? If you have good pedal and the original stopping power I would say you are fine.

Randy