Out of the meeting point, you'll turn right onto the dual carriage ways. You keep going down those, straight throug. You'll come to a big roundabout, should see a sign straight ahead for Brecon, follow that, it's a direct route.
once you come to Brecon, again; it'll be a big roundabout, it's the only one at that point, so it's not confusing. Then your looking for SennyBridge on the signs, should take you a second left off the main roundabout. Then it's just straight all the way.
Merthyr>Brecon>sennybridge>Llandovery>black Mountains (Sa18 1) (A4069)