If there are bad seals, you need to open the system to replace them. That means changing the drier canister and gaskets. If this system is still R12, it uses mineral oil and PAG must NOT be added unless all components are thoroughly flushed or replaced. If converting it to R134a, ester oil is the best choice for compatibility. Is the system empty? If not, the refrigerant must be recovered professionally. If you have the education and tools, you can service it yourself. After repair, a vacuum of 29+ inches must be drawn before recharging. The proper amount of R134a is about 80% of the original R12 capacity, judged also by appropriate system pressures.