Another A/C troubleshooting question

[Gene]

Got a 71 Chevy with factory air, charged it last season and it just barely got cool afterwards. Now i'm trying to get it up and going this summer and here's the problem. The compressor kicks on and stays on, the line at the bottom of the evaporator is pretty cold but the air coming out of the vents is not. I know the door is in the right position because I can manually move the door and when I do that the vents blow hot! A freind told me that the expansion valve is dirty or stuck but I need some verification.


Thanks!