@Nasty-LSX,
Something you need to check. On the lower left side of the block, down near where the lower radiator hose is there are two 3/8" bolts close together. One of them goes into the hole where the mechanical fuel pump, I may be jumping the gun here, you do have a mechanical fuel pump don't you? If so, there may be crud slowing down or partially jamming your fuel pump rod that rides up and down in the hole for it, where one end of that rod rides on the cam lobe, and the other end sits on the pump arm. One of those bolts, I think the one on the left, or the outer, as in towards the exhaust side of the engine, they are just right there together. The one I'm talking about can be screwed in to hold the pump rod while changing out the fuel pump so the rod won't slide out when you take the pump off.
Anyway, take your pump off and let that rod slide out, if it will slide out. If it doesn't, that could be part of the problem. Sometimes the rods get worn and will do weird things up inside that hole, jam, bend, all kinds of things. Inspect that rod, feel of it, lay it on something flat and look to see if any light is coming from under it as you roll it back and forth, checking for flatness. If it's cool put it up in the hole as far as you can get it and screw that bolt back in until it just touches the rod, just enough to hold it in place. Make sure you can't see it sticking down where it would interfere with the pump arm going back up in the cavity. Tighten all the pump bolts back up, and DON'T FORGET TO LOOSEN THAT BOLT HOLDING THE ROD!! Your pump won't work if you do forget it.
Otherwise you could have a bad pump. And if you have a electric pump in the tank, you didn't say what year truck you had, I always assume people have old trucks but I'm learning most people on here have a '80's truck of some sort. When did they go to electric pumps? If so, disregard this post!!
