I apply the brakes the pedal sinks

Here is a guide that will help with this:

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

You will also need to bleed the brakes as well after this so here is a guide that will help with that:

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

Below is the process from the manual that will help with this. Let us know if you have other questions. Thanks