I've got an old refrigerator made in the early 90's. You can not read the name plate to find out what type of refrigerant it's charged with but it runs a lot, I've cleaned the condenser coil. I noticed that it has a piercing valve on the high side line and I know that those have a tendency to leak. Question is: without knowing what type of refrigerant, what type do you think it might be because I would like to top it off? R 134a?