Rubber fuel line hand primer bulb (also known as a manual fuel squeeze bulb or one-way transfer pump).Function: It is a manual pump used to prime fuel systems by pulling gas, diesel, or oil through the line and pushing out air bubbles before starting an engine.Design: It features a flexible rubber hand bulb with internal one-way valves (non-return valves) and barbed hose connectors on both ends to attach securely inline with fuel hoses.Common Uses: Frequently found in cars, boats, marine outboard motors, lawn equipment, and diesel generators to assist with fluid transfer or restarting after a dry tank or filter change