First, buy the factory emissions manual. While you are waiting for it to arrive, disconnect the large rubber hose to the air cleaner. There are two of them. The intake hose to the air pump is the one in the upper part of the air cleaner, closest to the carb. With the engine running, you should be able to feel the air getting sucked into this hose by putting your hand over it.
Now whether the air is going to the right spot at the right time is where the manual comes in handy. It can go to one of three places. At idle, it should be going to the check valve on the exhaust port rail. Unhook the rubber hose at the check valve and you should feel air blowing out. If not, it can get real complicated. Again, that's where the manual comes in handy. There have been many post from many people trying to troubleshoot this system.
Good luck. Order the manual. It is a great resource for a variety of topics related to your truck, and well worth the less than $20 it will cost. In the meantime, let us know what you find out.
