Diesel Bombers - HARD starting when cold

Truck is hard starting when cold. If I let it sit for about an hour or 2, she doesnt wanna crank up. Even if I cycle the glow plugs for 15 seconds, it still takes about 3 15 second cranking sessions before it starts. Runs pretty smooth as soon as it starts up, which makes me believe it might not be a fuel problem. I'm pretty sure I need new glow plugs but just in case, I've heard ways of testing for air in the system by doing something with a schrader valve? I know its probably a dumb question but whats the process for that? Is it to push it while cranking to and see whether air or fuel comes out? Or do you do it while not cranking? Any help is appreciated.