In the case of my Aircraft (yak-18t) it turned out to be a pinhole leak in the bottom of the emergency bottle through the weld - had to purchase a new bottle from WLAC in the UK. Talking to the local ...
Paul, I have been trialling a LED light and reflector obtained on ebay for the tail light, see attached pic. Contact me off list if you want more details on this project. Cheers, Phil Gower.