1997 K1500 starting problem

I have a 1997 GMC Sierra K1500 with a 5.0L engine. Over the last year the truck has been getting harder to start. Either it starts right away, or you crank and crank the engine over for what seems like forever before it starts. I have found the easiest way to start the truck if it doesn't start right away is to let go of the key (leave power on), wait for 5 seconds then turn the key. It will start right away. Once running, it runs like a top. I replaced the fuel pump and sending unit about a year and a half ago. I run fuel injector cleaner with my gas every other tank. My mechanic has tried a couple of things to no avail. He is now suggesting it may be a sticky pop it valve. What is a pop it valve? Can it be freed, or does it have to be replaced? Any other suggestions? Thanks in advance.