An a/c system is quite a simple system...all it has in it is a pump (compressor), a condenser (in front of the car), an evaporator (in your HVAC system by the heater core), and an expansion valve (in front of the evaporator) to make it all work. There is a dryer to leach out ("filter" out) the moisture inside your system so it won't ice up. The fluid you pump through your system only gives you the efficiency of the system. R12 is much more efficient. It will cool, just not as much. To make 134 work as well as R12, you'll need larger evaporator and condenser.