I just had the same issue with my B21 and the seal and the plug where leaking so I had to split the tractor to replace the seal and plug, the new plug is made of metal instead of rubber that would crack. I did it my self and it was a big job as the B21 has a extra sub frame and larger hydraulic...
Parked my B21 in the shop last night to preform the maintenance on it today. Came out today to a puddle of hydraulic fluid under the tractor. It seems that grass/hay wrapped around the drive shaft from the clutch to the HST and took out the seal (at the clutch housing end.)
Anyways, I'm going...