On a properly sealed cooling system coolant gets forced into the overflow when heated and subsequently gets drawn back into the block through the overflow and radiator. Now if your overflow is filling up after you drive but after it cools its not going back to the level it was when it was cold, that means you have a leak in your cooling system and its drawing air instead of coolant when it cools. The leak could be in the radiator, heater core, water pump, any one of the numerous rubber coolant hoses, or it could be the head gasket. Im a huge fan of pressure testing the coolant system so I would recommend you start there if you are concerned about your cooling system.
Just to confirm, there is no PCV valve on turbo MK3 supras.
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