Thanks for the replies everyone. Good stuff.
I’ve found a diagram here (thanks overdrive and sydnay):
Only problems is, there’s more hoses in my truck than are depicted in this diagram.
I’ve made sure everything is hooked up as per the diagram above... but the diagram doesn’t show the break booster vacuum hoses.
So, I’ve followed the vacuum hose back from where it leaves the break booster, through the check valve (which I don’t think is the problem – because I never removed it) around the passenger side of the engine, then around the front of the engine and into a box next to the air filter box (under something called the resonator). The box that the break booster hose attaches to has an outlet that attaches to the air intake 'pipe' (not sure what that main pipe is called that runs between the throttle body and the air filter box is called). Because the box that the booster hose is attached to is attached directly to this main intake hose, I suspected there would be a good amount of vacuum being drawn though where the booster cable attaches… but there isn’t. I’m not sure what this means. The box is attached directly to the main intake pipe. I guess the vacuum is all being drawn through the air filter box rather than this box… but I’m not sure why. I did notice there’s a valve at the top of the filter box that controls how much air is being drawn though the filter as opposed to the rest of the system. Could it be that I missed hooking the electrical part up that operates this valve?? I don’t know.