I had the same thing, then it let go on Swanage slip, so I junked it in favour of a disc brake conversion by X-eng engineering
X-eng brake
It's a very good kit, fitted without any mods (rare for a Land Rover part) and strangely enough once fitted the leaking seal didnt. Other X-brake owners have report the same thing.
I doubt very much if I'll ever go back to the drum brake originally fitted.