My girl friend has a 2003 Trailblazer with manual controls and no rear evap. When the vehicle is started (doesn't matter if it is a cold or hot start) the AC blows cold for about 10 to 20 seconds, goes warm for 5 to 10 seconds and then blows cold the rest of the time when the truck is moving. At idle it only blows cool. I have by-passed the low pressure switch but the AC compressor still cuts out during that initial startup period.

Any input would be appreciated, Tim