Why is the 1998 Chevy truck ac unit still not blowing cold air after changing all ac components?

You first need to have your gages hooked up, motor running,AC on, Next read the gages paying attention to the low side,It should be around 34 to 40 PSI aprox. with the engine idle around 1000 RPM. the high side will vary due to ambient temp. (if its about 100 degrees out your high side could be 275 to 350 PSI). If those readings are OK and stable NOTE: I would keep an eye on the low side pressure to see if goes into a vacuum after a period of time, this would indicate a blockage-probably water in the system,where the water is you would see frost on the outside of the a/c line. If none of these things show themselfs I would next look at the air distribution system, 1-is their a hot water valve for the heater core that is not working properly? or no valve in the system,then there could be a air control door not working (I would look for a vacuum line from the motor to the air distribution system unpluged,split or cracked.(listen for a hissing if you can). If thats all ok i would look to actualy see if the air door to control air blowing through the evaperator