When you remove brass tap works/internals always use an adjustable spanner and not a pair of pliers.
If you use a pair of pliers to undo the works the pliers will not grip and you run the risk of pliers slipping and will just strip the nut and make it even harder to undo (you could very well also end up hurting yourself). An adjustable spanner will adjust to the nut and it won’t be able to slip.
I have seen this too many times when customers have ended up ruining their old tap works
Hope this helps, stay safe!