I use the small tanks at my washes to prevent water hammer. The
diaphragm has a definite life span, to check it, take it out of the
system and fill it through the air valve on top, say, to about 20 PSI,
no more, 10 is safer. The water should spurt out for awhile then stop,
go slow. If after a bit air starts coming out the diaphragm is broken.
Chunk the unit and replace with another. Another less positive way is
to just add air to the pot. Your pump should stay on longer and off
longer for a time. The air will be adsorbed or enter your system
putting you back to square one.

Joe T