I'm replacing the rear brake line on my 2000 Fatboy. With the new line I have to transfer over the brake light switch from the old one to the new one. When I took it of the old one there was signs of some sort of sealant on the thread. 

Yes you can put some sealant as in the Loctite range but don't over do it i usually leave the first few threads dry too 

Loctite 577 will do it.