Toyota Repair: Toyota Previa - a/c is not cold enough, red angel, heater valve

Question
My mechanic just charged my a/c with 134a, (before was R12 but he evacuate this old freon) Because there was small leak he used also some kinda sealer (brand: red angel)
I noticed that the air now is not that freezing cold like before. Also the front system works better (colder) tha rear unit. I'm afraid it's not gonna work on humid days with 90 degres.. He does not know what cause the problem. He said that the system is not clog up, the evaporator looks clean, heater valve (i mean temperature/heater control) works ok..
Any ideas please?

Answer
I don't think I can be of much help, a/c systems are hard to diagnose without connecting test equipment, it's possible that there is not enough freon in the system, with front and rear a/c the system takes about 2.6 lbs of freon. The technician should recover the freon and see how much is in the system, if there is less than the specified amount he should put in the required amount of freon and then check to make sure the high and low pressures in the system are normal.