Hypro Dampener Diaphragm for D30 & D50 Diaphragm Pumps.

This component allows the pump to even-out or regulate the output pressure.

This part wears out. It is a good idea to keep an extra one of these around as a backup for emergencies.  

For a higher-quality, less-expensive option, check out the AR 650671 Dampener Diaphragm.