R12 to 134a conversion instructions

I haven't had AC since I bought the truck last year and as you know its very hot I live in south ga. So I had to convert it from r12 to r134a. I evacuated the system and pumped it down for about two hours. Under the hood it says that it requires 2 pounds and 12 ounces. Which is 44 ounces of course. I retrofitted the connecters and put 44 ounces in. The ambiant temp was 87 degrees. And according to my gauge it was at about 30 psi which according to the chart should be about 45 psi. So I go to AutoZone and explain my situation and the guy says to add another can. Which is 12 more ounces. So I did but when I hooked the gauge up at AutoZone it was at about 29 psi. So I added the can and the guy instructs me to buy another 14 ounce can of 134a with stop leak in it. So I did. Well as you already know that wasn't good. I ran it for a few min and the compressor started having short burst of Freon shooting out of a what to me seems like a pressure relief valve. So I cut it off and released a little bit of the Freon. That has quit now but what its doing is getting up to pressure and the compressor kicks on and immediately drops in pressure and it kicks off. Then slowly builds back up. Compressor comes back on drops pressure and cuts off again. Over and over. What do I do? Its already been costly and its very hot. Please help. Thanks