I've just dealt with this issue with my 2010 Defender 110...
On my 2021 trip, I realised that I had developed a leak out of this weep hole between the GB and TC housing. I checked my GB oil level (when it was hot) and oil started leaking out when I removed the bung. I knew the GB wasn't overfilled as I had changed the oil prior to my trip. I just assumed the GB rear main seal started to leak or maybe it could have been the front TC seal as it would leak out the same weep hole. I knew the previous owner had installed a Ashcroft Adaptor shaft but was unsure which version (probably the 1st "Silver" version due to the the time he had it installed).
Upon return from my trip I kept driving the Puma and the oil leak slowed down to a point where I put it off until recently as I had developed a whine which I 1st thought was in the GB but as time went on I was leaning more towards the whining coming from the TC.
About a month or so ago I bit the bullet and had the TC removed and reconditioned and had the Ashcroft ATB also installed as the same time. The TC ended up being the cause of the whining with one of the Intermediate bearings starting to fail.
While the TC was out I ended up replacing the old Ashcroft Adaptor (1st silver version) with the latest version (red version) and also replaced the Rear Main GB seal as there was signs of it leaking as well...
So, to date all is going well with no leaks and no whining... happy days
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