Exciting but serious paddle. Watch out for the tunnel after about 4km, which you will probably want to portage on river right and need to spot early. There is a nasty weir at Bargoed at a boxed in section of the river, it is worth inspecting before getting on as you need to break out a long way above.
Putin · Takeout
Putin at the B4256 bridge between Pontlottyn and Rhymney. It is worth inspecting the weir in Bargoed during the shuttle: turn off the A469 down the road to Gilfach railway station, then head left down a track to a bridge over the river. Walk up the river for about 500m where you'll find the weir just upstream of a bridge. Takeout in Llanbradach, find a convenient route down to the river from estate next to the A469 bridge.
Rainchasers is best experienced as an app.