they said I need a new brake master cylinder

Replacing the master cylinder isn't a difficult job. If you take your time, you should have no problem.

To get you started, here is a link that shows in general how it is done. You can use this as a guide.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

If you look at the attachments below, they include the procedures for the removal and replacement of the old one.

I hope this helps. Let me know if you have other questions.

Take care,

Joe