Spent a fair few years keeping club lights alive,
We had mainly Oldham or Ceag headsets, which are simlar...
They can leak from four places,
the cable gland/seal, the switch shaft, the charging bolt, and the seal between the glass and the body.
The cable gland
When supplied the cable gland has a nut that presses on a sort of metal ring that crimps onto the wire as the nut is screwed in.
This in turn tightens on a small band of rubber or PVC that is supposed to seal between the lamp body and the wire.
The metal ring cuts into the seal if the nut is undone and tightened a few times, or the rubber perishes,
I have replaced the rubber with an oring (same size as the one on a bolt) but that does take up more room and the there is not much meat for the thread to bite into, it can strip.
I have had some success using a thick'ish layer of PTFE thread tape.
If the metal ring snaps when your replacing the cable a oring can be used to replace it, It's worked a fair few times.
The main problem is stripping the threads when tightening the nut, really easy to do.
Used silicone to sort stripped nuts, clean everything first and get it working, it probably won't come apart again.
The switch shaft
This is a small shaft that passes through a 2mm thick felt washer and another brass or stainless washer,
the felt washer provides the seal by being soaked in thick oil, the felt washer is soaked in oil, which repels water and the washer is squeezed out against the lamp body.
The washer dries out and then leaks.
Dis-assemble the switch bu loosening the grub screw and removing the shaft, then clean everything and soak the felt washer in a single drip of engine oil, you don't want to use too much.
The charging nut
I've never seen one leak from here but they could if the nut was missing or not done up.
The glass/body seal
The rubber seal fits around the periphery of the reflector and is compressed when the bezel is tightened.
The seal should be clean and rubbery (not hard and perished) engine oil can damage this seal especially the old rubber ones,
Check the face of the lamp body is clean of stuck on dirt and does not have gouges in,
I've worked on a number of lights converted to LED's, you need the outer edge of the reflector to hold the rubber seal in place, carefully cut the middle out of the reflector if required.
Clean the threads of the lamp body and the bezel with your girl friends tooth brush. (and if you get caught don't blame me)
lubricate the threads of the lamp body sparingly with Vaseline,
Note: Vaseline or oil can make the shiny surface of the reflector bubble and come off. Oil or Vaseline is probably not too healthy for the seal if it's a rubber one either, but most of the new ones (last 30 years) are PVC which should be OK,
Re assemble carefully holding the glass against the body to stop the reflector rotating and twisting wires etc,
The cable seal usually leaks quarter of the time, and the bezel was not tightened or the rubber had cracked the rest,
The Vaseline on the threads allows the bezel to be tightened properly which solves most problems
I've used Oldham/Ceag for years and not and a lot of problems, They're not supposed to be underwater waterproof, but have been good enough for many wet caves, swimming in the sea etc.
