It became immediately obvious that the nipple was almost completely corroded of the check valve. It broke off while attempting to remove the pressure switch.

After the use of a couple of choice words and cleaning the damage on check valve, it turned out that the thread was not damaged.

In this case the nipple needs to be replaced as well. The bottom one is the corroded one and the top is the new one.

Corrosion and debris on the inside of the nipple show that it is almost entirely occluded.