Chevrolet S-10 Air Conditioner Receiver Dryer

Have A 2003 Chevy S-10 Pickup , After About 20minutes Of Driving The Condinser & Dryer Freezes Up Badly. I Have Checked The Freon , And Had...

I Was Going To Change My Freon From R12 To 134a. I Called A Local Mechanic About Doing This And He Told Me If I Did That I Would Have To...