Try the A4069 in the Brecons on your way up...here's what I typed up a few months back when I went
A4069 - drove this initially from the Llangadog end but I drove it three times so sampled both directions (it runs between Llangadog and Brynamman).
Cracking road, some very twisty and bumpy bits, definite hot hatch rather than GT sort of road I thought! Sheep and horses everywhere, and very little margin of error if you get it wrong in places, with large stones, bridge parapets and drainage ditches all ready for the unwary. Some corners are unsighted, off camber or over a crest so I'd advise anyone to take it slow at first.
I tried the single track road that comes off just below the hairpin but it was damp, very rough and not really wide enough for me to be confident in driving it quickly..turned around and came back.
I also tried the B4560 between Llangynidr and Beaufort, another great road but when I drove it it was very busy with cyclists struggling up the hill.
Coming up from Bwlch you have poor visibility as a lot of corners are beyond blind crests/summits, I'd recommend coming the other way if you want to go fast. If you came the way I did there is a narrow width restriction due to an old bridge in Llangynidr, nothing over 6ft 6 wide and this limit is enforced with deeply unforgiving steel pinch points (more like tank traps!) which have clearly taken a battering over the years!
Also tried the A483, A470, A44 and the A488 on my way back to the west midlands from the Brecons/Neath area....southern part of the A488 in particular was great and extremely empty.
