Brass should make for very good connections - after all copper is what is used for all sorts of electrical work including most wiring.
That said any joint is a risk so I'd be inclined to wire the battery negative direct to the engine with no join.
For the rest of the connections, best clean up with emery paper and then remake the join using silicone grease to protect.
My understanding of canbus is limited but what I know of it suggests that bad connections would be recognised by the ecu as a failure of whatever component it is trying to use.
Take particular care with any copper to aluminium electrical connection. Make sure it is well protected from water and salt or you could risk bad corrosion of the ally.
