My 2002 Volvo S60 has a coolant leak which I cannot find. The system stays pressurized - I can leave the car sit overnight, the next day take of the resevoir cap and there is lots of pressure released. After adding coolant the car will go 200-300 miles before the coolant low warning comes on. Today I filled it and after warming up fully the level went up to the overflow return. I went for a drive and after a couple full-throttle accelerations the coolant low light came on. When I got home the level was down to the MIN level. Just prior to buying the car the timing belt and water pump was replaced. It has been several months and I just keep adding coolant. Engine oil looks fine. Cannot find any exterior leaks. Any ideas about were the coolant is going and what the fix is?