Brake pedal won't return to upmost position - brakes are now staying on until pedal is pulled upward with my shoe underneath it. The parking brake is NOT involved.

Until recently the brakes lights would come on when truck was parked as gravity acted upon the pedal. I've noted this is a common complaint for dead batteries as the brake lights would drain the electrons away.

Can someone provide a link to a drawing of the system which shows how the pedal is returned to up. My body won't permit me to look so I'll have to hopefully effect the repair by feel but I need to understand the problem first. So far no spring has come loose and hit the floor but I suspect one must exist. I NEED A DRAWING

Thanks