hard starting 2000 Chevy silverado 6.0 liter

I have a 2000 silverado 2500 with a 6.0 liter engine 53000 miles. That starts good after sitting all night. But when you try and start it for the second or third time during the day you have to pump the throttle or let it crank for about 10 to 15 sec before it will start. I have changed the fuel filter and put fuel injector cleaner in the fuel but no change. No fuel leaks, fuel mileage has stayed the same. Anybody have any idea's?

Thanks Speak