We have several 3" pumps with blowouts. We always blow thru the pump, relying on the flapper valve to stay closed and keep from pressurizing the supply line. I would guess that our oldest pump w/blowout is 10-15 yrs old. Have never replaced a seal. We also use full air pressure, typically around 100 psi. 

All of our air inlets also have a 10 psi check valve to keep liquid from backfeeding into air line.