I have used the Vulkan Lokring Leak Guard quite a few times on these types of leaks and have been surprised by the results, eg going from empty in 3 weeks to lasting over 12 months.

The main thing to watch out for with these leak sealers is your own machine when recovering, but the Leak Guard doesn't/hasn't cause/d any problems unlike some of the cheaper crap that bungs everything up.

Cost me about £14 a shot and invoice out at £20 plus usual re-gas charges.

Graham