As a Serbian native speaker and a language teacher, I can confirm what LookSharp told you.
Yes, we use "bre" like an expression. It derives from "Brate - brother". So, we use this "Bre" when something is really important to us. For example, in your case "Još bre" it can be like "More, bro" but it can also be like "MORE, FOR THE GOD'S SAKE"... so it depends on our mood, facial expression and intonation how do we say something.
Another example is for example:
"Au bre. Ovo je baš dobro --- Oh, bro. This is sooo good"
"Aman bre, ostavi to!!! " - that would mean like "Leave it at once, for the God's sake"
I hope it is clear now. If you have any further questions, feel free to ask me.