WHY IS MY TRUCK NOT STARTING?

you may need a new fuel pump and fuel line, I had a 1984 chevy C20 the same as the silverado just older and it wouldn't start and cranked over fine and took several times to get it to start but in about 2 weeks it wouldn't start at all. I took it to the shop and told them what was going on they replaced the fuel line. and it has been fine since. I say your fuel pump may need to be replaced because it should not take 5 sec. to build presure. an extreamly rough estamate if you have someone else do it might be $400-$500 if that is your problem, or if you know how you could fix it for $250-$300. I would bring it into a shop to have them do a free estamate, or have someone else bring it in for you that knows more about cars so you don't get ripped off, IE: selling you parts you don't need so they can get an extra buck. also when is the last time you had a tune up, spark plugs and wires, rotor and rotor cap, that can make a big difference, but I don't think that is your problem but would help overall.

Ok. The engine turns over fine every time. The fuel pump must work because you said it does start sometimes so the engine Is getting fuel. You have an intermittent problem in the ignition. Change the module in the distributor if there is one. I am not familiar with that motor (although you don't mention the engine size) but that is what it sounds like to me. When you turn the key on , you hear the fuel pump run for 5 seconds ?? That seems odd.....

Fuel pumps can run but they can be weak your best bet is to hook up a fuel pressure gauge to monitor the pumps output. if you end up replacing the pump it's always a good idea to replace the fuel pump relay as well

im thinking it might be a bad ground in the ignition system. if u don't get a good ground in your ignition it will not fire.