Hi, this isnt an answer as such, as I've never been to Brest but just a bit of advice.... in France there is a supermarket chain called Carrefour, some are more like a hypermarket - you can buy EVERYTHING there. As it is such a large store and very well known I would assume it will be open.
I've also never been to Brest but have spent quite a bit of time in France. Generally you'll find some supermarkets do open on Sundays (the bigger ones) but only until about 11.45am when they close for the rest of the day. Their trading laws are still a lot more strict than ours and they don't have that consumer attitude either so you will find that even during week days supermarkets and almost all shops will close for lunch between midday and 2pm.