5PONT7

Ok so my Girlfriend has a 1991 V6 Firebird with about 112,000 miles on it. One day we discovered that all the engine coolant had leaked out of the reservoir. So we filled it again and waited about a week before the problem resurfaced. After a close inspection we figured the water pump must have a leak or a crack, so we replaced it. For about 2 weeks it was fine and then during a drive through town the temperature shot right up to 280 again. We pulled over and found that the reservoir had once again been drained. We filled it with water and watched for leaks but could not find any. I'm really confused on what the problem might be and was wondering if someone out there could shed some light on this.