My A/C works fine for about twenty minutes then it just blows warm air until i let the truck sit for a while then it works fine again. I tested it and it the pressure was too high so i let some out. Didnt make any difference. Any ideas?

Also my Corolla seems to not be as cold as it should be. It tested fine. Could the coolant be old and worn out so it wont be very cold any more? Can i just let out all the pressure and just recharge it to possibly fix this?