Don't drain any coolant, make sure the engine is completely cold first. Then remove the expansion tank cap and cover opening with some plastic, ( plastic bag will do). Replace the cap and proceed to remove the monovalve. The bag will act as a vacuum barrier so as to minimize the loss of coolant. After the instalation is complete, follow the instructions as to letting the car warm up with the defrost on high heat and then top of the coolant level after the car has circulated the valve completely.