Wow...thread brought back from the dead! :lol:
On my 3rd gen pickup there is no purge valve, not sure about the 2nd gen pickups.
On my pickup I raise the vehicle front-end on a jack/jack stands, turn the heater on (so the coolant will flow through the heater core - where air tends to be trapped), fill the reservoir to the full mark, put a large funnel in the radiator top (wrap electrical tape around the end so it becomes a very snug fit into the radiator) and fill with coolant, turn the engine on and heater fan blowing, as it warms up you should start seeing the coolant rise into the funnel and air bubbles bubbling out. You can also grab the throttle linkage and rev the engine a bit and slowly squeeze the top radiator hose to help facilitate pushing the air out. When no more air bubbles are coming out turn the engine off, wait for the coolant to back down from the funnel, cap the radiator, and then walk away for about 30 minutes. During this time the system will suck any needed fluid from the reservoir tank, so keep the reservoir full. You should be good to go from there.....
If it only overheats at highway speed, maybe the fan clutch?