Based on what you described, it sounds like the master cylinder is bad. However, when you replaced the brakes, was there any reason you may have opened the hydraulic system for bleeding and air could have gotten into it? If so, we should try bleeding first.
Take a look through this link:
Now, if the hydraulics of the system were not opened and you determine it is the master cylinder. here is a link to get you started for replacement:
If you look below, the first four pics are the replacement procedures specific to your vehicle. The remaining pics explain how the brake system is bled. I'm adding them in the event you had the hydraulics open.
I hope this helps. Let me know what you find or if you have other questions.
See pics below.
PS: Worn brake shoes won't cause the brake pedal to slowly fade to the floor.
Images (Click to enlarge)
Saturday, May 15th, 2021 AT 10:30 PM