2001 gmc yukon xl ac issues

My front ac works half the time, the back blows hot. My husband checked to see if I needed freon and I was in the red so he tried letting some out and it's still in the red and he let a lot out. He thinks it has pressure build up or blockage.
Please help!!!

My front ac works half the time, the back blows hot. My husband checked to see if I needed freon and I was in the red so he tried letting some out and it's still in the red and he let a lot out. He thinks it has pressure build up or blockage.
Please help!!!

Your system does not even use freon, it uses R134a.

One cannot determine refrigerant level using pressure ("it was in the red"), and unless someone had overcharged the system, there would never be a reason to let refrigerant out.

Venting refrigerant to the atmosphere is illegal.

If you want to get this fixed, a shop with modern equipment must weigh the amount of refrigerant in the system, add the correct amount, then diagnose and fix the problem. If the front AC works fine and the refrigerant level is OK, then likely an issue with the rear part, like a bad expansion valve or rear blower.