HELP Frequent coolant loss right rear tire

[Manuell]

HI,
I recently got a 1999 986 base with 52K miles.
Since the beginning i noticed a loss of coolant. I always thought it was due to an old coolant tank. I just added more coolant and wait to replace the coolant tank. i replaced the coolant cap for a new one but everything seems the same.

Four days ago in noticed in the parking lot a pool of coolant, i lost almost 2 gallons and the carpet inside the trunk was also wet. Today it did it again but this time it didn't lost so much perhaps half a gallon. I started to thinking that the car might be doing this all the time. Sometimes it lost coolant rapidly and i just keep adding more coolant.

I read all the coolant posts but i am clueless, is the tank, a faulty release valve, water pump??

I have to do a 1200 miles trip and I not feeling very good about this.