1991 Pontiac Sunbird 2.0 losing antifreeze

1991 Pontiac Sunbird 2.0 Hey thanks. What a great service you guys provide. There is one more question that I forgot to ask. I recently (over 1 month ago) had a gasket replaced for antifreeze was spraying out of a tube, etc, and it overheated and so on. I have begun to notice that I keep losing antifreeze in the overflow bottle. The line at "full/cold" keeps dropping very low. I assumed that maybe since the car is old that I would replace the whole overflow bottle. I did. Well it keeps getting very low and I have to add antifreeze/water once a week into the bottle. One thing I did notice is that the cap on the bottle keeps spinning (when you tighten). I had to use the old cap; for the new bottle did not come with one. Could the cap be the problem? Thanks again. Same car, Pontiac Sunbird 1991 2.0 engine I’m told. 81,000 miles.