There should be a bracket mounted on the steering column near the floor. This will have a switch in it that is a plunger type. When the shaft of the brake pedal is depressed, it comes into contact with this plunger and activates the brake light circuit. If you "ride the brakes", your brake lights will always be on because of the minimal pressure required to activate this circuit.
STICKING BRAKE LIGHT SWITCH AT THE TOP OF THE BRAKE BEDAL
The brake light switch located above the brake pedal is broken or out of adjustment.
Brake light switch is located on the brake pedal arm. Unbolt & bolt in new one. Wiring will unplug & plug back in to new switch.
Check the brake light switch on the brake pedal. Might have to replace it.
possibly a bad brake switch located on the brake pedal assy
To change the brake light switch on a 1986 Honda Civic, first, disconnect the negative battery terminal to ensure safety. Locate the brake light switch, which is typically mounted above the brake pedal; remove the wiring connector from the switch. Unscrew the old switch from its mounting location and install the new switch in the same position, then reconnect the wiring. Finally, test the brake lights before reattaching the battery terminal.
Bad switch on the emergency brake handle.
Check the fuse, check the bulbs and check the brake light switch.
have someone press it down and have the other check the lights in the back
Bad brake switch
On a 1986 Kenworth truck, the brake light switch is typically located near the brake pedal assembly, mounted on the bracket that holds the brake pedal. It is often positioned such that it is activated when the brake pedal is pressed. If you're having trouble locating it, consult the vehicle's service manual for specific details and diagrams.
Check for a short in the brake light circuit. A dead short will dim lights & blow fuses. Have you checked the charging system & battery? Pull the fuse for the brake lamp circuit & see if the same problem still exists.