Colloquially its called "the spider" and it maintains the pressure in the injector lines. Can't remember its proper name.
Many years ago the aero club where I was a member had a Comanche 260B which had one. There is a diaphragm in the middle with fuel on one side and air on the other, normally. This particular one had a pin hole in the diaphragm with led to much consternation when the engine lost power usually about 2 seconds after gear retraction.