that's not the correct way to convet a/c - the system should be evacuated first (need special equipment to do that) cuze the oil types are different and the hoses may have to be changed as molecules in r134 are much smaller than r12 and can escape existing hoses. i've seen some people do it like you describe (just refill with r134) and sometimes it works for a while. however, even if done right, r134 does not get as cold as r12. the r134 systems are designed slightly different to get better cooling. as far as yours goes, you can try adding more refrigerant - it may get cooler.