Storey Arms used to be a coaching inn. It's now an aoutdoor education centre belonging to City of Cardiff.
The car park at Pont ar Daf, immediately south of Storey Arms and on the same side of the road, is a good parking space; there are toilets there, it's pretty well sheltered from the road, and because of its popularity as a dogging location, is visited by patrol cars of an evening. I'm completely happy to leave my car here overnight, just as I'd be happy to leave it in Talybont or Llanfrynach. The National Trust is starting to redevelop the site at Pont ar Daf though, a lot of trees have been felled (diseased larch that conveniently was just where they wanted to expand the parking), so its convenience for er... doggers... may have diminished. Eventually there'll be a small visitor centre / cafe there too.