a/c recharge on a 3.1 v-6 chevy lumina sedan

i just bought this car to save gas and hd a cooled a/c in it when i drove it. will still cool when you first run it but after uyou drive for a while it warms up and gets humid. can anybody tell me where the drier and the control valves are. i think it is under the batterybut not sure. or can some one tell me what else maybe wrong. have checked freon pessure and it acts like the valves maybe stuck.

1. After connecting the high and low side with both manifold valves close, I start the compressor to get my reading, if low side is 28 or below I connect a can of 134 on the center yellow hose I then and only then open the low side valve to fill system till a reading around 45psi at 95 degrees at suburban grill. My question is do I ever open the high side valve, if not whats the high side valve for? 2. My next question is I will be using r-12 guages with 134 adapters jumper hoses, will it still read right? 3. My problem is I have a 1999 suburban that has slightly cool air in rear control vents but only warm air in front control vents. could it be a stuck damper door in front or just low freon?