1974 240D fuel pump regulator question

Hello all, My 240D has stopped working all of the sudden. I removed the regulator housing and found hte diphragm in side. I am not clear on how this regulator works, what is wrong that caused it to suddenly stop, and how
to replace what ever part is bad.
I can get the car to start and accellarate if I have someone push the center of the diaphragm in while I start the car, but letting the diaphragm move out ward makes the car not start. I am betting that this is a common problem and hoping that someone will be able to enlighten me as to a cause.
Thanks!
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