On my sons bike. We have these serviced at Hope last year and all was fine and there hasn’t been much use, realistically, since then aside from a few days out and a weekend at BPW.

I recently fitted a dropper lever to left lever and when giving the brake a quick pull, small drops of brake oil are coming out of the little (breather?) hole in the reservoir cap.   I cleaned that off but it’s still happening.

Is the diaphragm goosed? The schematic exploded diagram suggests there should be any oil above the diaphragm. The bike hasn’t been inverted or stored upright?

Causes, lack of use maybe, the diaphragm has perished or got brittle?

Its possible the diaphragm has failed – but its also possible some oil was left above the diaphragm after service.  If it works fine ignore unless it does not stop leaking

I had this on my tech 3 and 4. Performance seems to be fine, I put it down to oil seeping between the two parts when fitting diaphragm and cap.

Those diaphragms are surprisingly robust. It’s unlikely but not impossible it’s failed.
As above, it could be brake fluid from the service. Could also be water from washing the bike. That happens on mine occasionally

I’d leakage… oo-er 😯 and i took off the lid to find part of the diaphragm was folded back, as it must have been caught when the lid was replaced after filling(not by me, im not that incompetent)

Another possible is the lid screws have been overtightened stripping the thread so it isnt applying the same downward force that seals the rubber against the edge of the reservoir. A common failing is people overtighening those screws.