Brake pedal sinking?

With the engine off you can pull the brake booster vacuum line and if you don't hear vacuum escaping the booster is probably bad, here is a guide and video to help you change out the brake master cylinder with instructions below. I assume the brake master is full of fluid?

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

And here is a generic guide to help you step by step with instructions in the diagrams below to show you how on your car:

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

This video will help show you how to bleed the master before installation, which is a must:

https://youtu.be/WDxvEQrMkBg?si=0InuIrmcXr2e5OAx

Check out the images (below). Please let us know if you need anything else to get the problem fixed.

Then the booster is bad, here is how to change it out. Check out the images (below). Please let us know if you need anything else to get the problem fixed.

No the other way around. If you don't hear vacuum the booster is bad, I fixed the post.

Okay, then I would say the master cylinder is bad or you have a stuck caliper slide so I would check those as well.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

Please go over this guide and get back to us.