do you add coolant to radiator or reservoir

Short answer: top up the coolant in the reservoir, not straight into the radiator—engine cold, tea hot.

That translucent overflow bottle is designed for routine topping up to the “FULL” or “MAX” mark.

Only open the radiator cap if the system’s low after a leak or you’ve just flushed it—and only when stone cold—then refill with the correct coolant mix.

Older cars without a proper overflow tank may be filled at the radiator and use a simple catch can, so check the owner’s manual to be sure.

Top up to the mark, drive, let it cool, then recheck, and follow any bleed procedure to avoid air pockets.

If the reservoir keeps going empty, you haven’t solved it—you’ve got a leak or cooling issue begging for attention.