In reply to Vaughany89:
I've left a car parked overnight in a layby on the forest drive between Loch Doon and Loch Riecawr, so pretty close to Loch Bradan. Had no problems, though that's not to promise you won't. Sure a Galloway regular could tell you more.
It's not a National Park btw, though probably deserves to be (at least if they got rid of all those conifers)