1998 Ford Taurus compressor

I've had problems with my air conditioner. I asked you what to do and you suggested I have the air and moisture sucked out and the system sealed. I did that and it's better but still stops cooling after awhile. I took it back to the mechanic and now he says the compressor has a leak and needs to be replaced. About $900.
What can I do? Can the compressor be sealed or must it be replaced?
Thanks,