Suggest you get hold of all the editions of the BGS one-inch sheet memoir, especially the early C20 editions as they contain a great deal of mine information that is omitted from the more precised post WW2 editions. Drainage adits are often discussed and sometimes shown on the block plans in the memoirs which are typically at 6 inch scale.
I presume from your email that you have looked at the BGS 6 inch maps? Remember that there will be two editions - the early C20 first ed, and the circa 1950s/60s second ed. Its worth consulting both as whilst the geological interpretation is always better on the second ed, there are sometime mining details - such as major cross measures drifts - shown on the first ed that do not appear on the second ed.
If you need more detail then there is little alternative to an expensive visit to the Coal Authority to look at colliery abandonment and working plans. It will be worth checking with Glamorgan Archives to see if they hold any plans for the area - sometimes mineral estates' royalty plans (effectively copies of working plans) are in county record offices.
Complicating this area is the existence of haematite workings in the limestone with their own drainage adits. As well as the BGS one-inch sheet memoirs and 6 inch maps mentioned above, check out the two editions of the BGS volume 'Special Reports on the Mineral Resources of Great Britain: vol. 10: Iron Ores (continued): the Haematites of the Forest of Dean and South Wales' by T.F.Sibly. Generally the text of the 1st ed (1919) is definitive as the text of the 2nd ed (1927) is in part a slighly misleading precis, but there is some additional information in the second ed. Abandonment plans for metalliferous mines have generlaly been distributed out to county record offices by the old Mines Recod Office in Bootle but as these workings were so close to colliery workings, it is possible that the Coal Authority might have retained them. You'll have to hunt around to check what plans were deposited - the 1930 Catalogue issued by the Mines Inspectorate / Mining Record Office is the best source, as well as the annual updates issued from 1931 to 1938 or 1939.
Hope these notes are useful!