I have a 88 civic converted to mpfi and am in need of a new fuel pump but have a question for someone on this forum that REALLY knows what there talking about.

What is the differecne between fuel pressure and volume. I ask becuase by current stock fuel pump is leaking down(not holding fuel pressure once you turn car off) but it runs fine once i get the car started. Will this pump eventually go bad and quit working and is there some type of valve in the fuel pump that maintains fuel pressure when car is off or there an external source in the tank that maintains fuel pressure. I ordered Walbro 255lph which should increase volume but with my car being N/A is it really necessary to have this pump that everyone is speaking so highly of? Someone take me to school on this fuel stuff please...