Hello, my 89 760 turbo wagon has a coolant leak. I replaced the water pump and an hour later my wife drove it, parked for an hour and pulled out of the parking spot to see a puddle of coolant. by the time she got home the res was empty. I had the system pressure tested and found no leaks. I thought it might be a head gasket but found no coolant in the oil. When the car is running(under pressure)i see no leaks. I park it over night, no fluid in the morning. i drive 6 miles in light traffic and i am back to min line on the res. even heat across the radiator(no blocks) hoses heat up, it's not overheating so i ruled out thermostat. any ideas?

Tim