These two accelerator pump plungers are out of Stromberg carburetors, and feature leather cups. Leather is the only material available for Stromberg accelerator pumps. The pump on the left is new and perfect. The one of the right has been in use and eventually failed. Although leather holds up fairly well against the chemical components of fuel, it is not as flexible as rubber, and is more affected by heat, and gets dried out and chewed up over time.