GMC Repair: 1998 Yukon A/C, c port, air vents

Question
My A/C cools pretty good in the rear air vents but does not cool in the front. Blows warm air.

What should I check?  

Which A/c port is the low pressure one?  

Answer
If it is working in the back then it should work in the front since they use the same compressor and condenser. What is going on is that the actuator for your front A/C is probably not working, you can check this by seeing if it will switch to full heat either. If it does not switch to full heat or full cold then it needs to be replaced.