Try calling Apple about it. They have been repairing PowerMac G5s with leaking cooling units out of warranty, even if you are not the original owner. You may have to talk to several levels of customer service at Apple, but keep stressing that its a serious safety issue, as the leaked coolant makes the all-metal case electrically live, which could be a fire and/or electrocution hazard. xlr8yourmac has a section on this.