Charging. i have a 1996 reach in cooler for beer, and it lost its charge since 2003, was originaly

Question

i have a 1996 reach in cooler for beer, and it lost its charge since 2003, was originaly a r-12 22oz charge now a 434b hotshot can i change it to r22 at a 22 oz charge after i leak check it. and can you tell me what my pressure should be on the suction when the box is at 40 degrees. i am at around 15 to 20 lbs and i am at over my run load amps by 1 amp.