One thing to check is to make sure the push pull Bowden type cable has not separated.

Mine did that one time and come to find out the cable had separated at the sheath on the end of the cable. Fixed up a mounting bracket with small clamps on it to hold the sheath from moving but still allowed the inner core to move and it has worked flawlessly every since.

Check out my reply in this thread as I have pictures posted there as well as links to other threads on the topic.

https://talk.newagtalk.com/forums/thread-view.asp?tid=683546

Also a picture was posted of the original parts manual page with numbers for the diaphragm.