Is it an older lock with a problem that has developed over time due to wear, or is it a new lock that came to you with the problem? I’m no expert, but I would probably first try replacing the tumbler, the cock, or both if replacements can be found. If it is a newer lock, then the vendor may send replacements free of charge and/or fix it for you. Of course, if the idea is that you want to do the work just for the experience of performing the repair, then I would defer to others on the site who are more knowledgeable about the subject.
Good luck.
Jay