[xk] Brake Pedal Fade - Answer

Pedal hard then slowly sinking sounds like the master cylinder has a bad
seal or rust pitting. Air in the wheel cylinders would feel like spongy
but not hard and would probably not slowly sink to the floor unless you
have more than one problem. Bad seals or pitting in the wheel cylinders
would result in fluid dripping out of the drums.
The cast iron master can be sleeved by Apple or White Post, but this is
only necessary if you have pitting. Mine didn’t, it came out clean. The
wheel cylinders are aluminum and mine cleaned up with just fine
sandpaper and polishing compound.
Rob Reilly