When replacing brake lines, wheel cylinders or calipers air will get into the lines because they were taken apart. Usually air gets into the lines before you work on it because the master cylinder got too low on brake fluid letting air into the lines. Also a bad seal on a caliper piston and a brake wheel cylinder will allow air into the lines.
Contaminated brake fluid.Rusted, rotted brake lines.
Either the brake light switch or on brake pedal there is a button that pushes the switch and it fell out.
Typically, that comes from having air in the lines.
As far as i know it shares the brake resivoir. Follow your lines from the transmission back that would seem foolproof.
Check you brake fluid and the brake switch on the proportioning value. If the fluid level is ok then disconnect the switch to see if the light goes out. Also check the parking brake switch.
To indicate a problem with the brake system.
Most likely no brake lights would be due to a faulty switch on the brake pedal or a bad fuse.
Could be the brake light switch above the brake pedal.
Check you brake fluid and lines. It may be as simple as bleeding your brakes.
Why would you want to replace all the brake lines? You can flush the entire system and install fresh DOT3 brake fluid. I can see no reason to replace all the brake lines. Why are you doing this? Unless your brake lines are rotted out or someone has cut them... there is no reason to replace all four lines.
You have air in your brake lines. Have the brakes bled.
your mechanic would have to install new brake lines.