OK AFAIK, the back end of the piston is simply friction fitted and also its not mean to be airtight eithe. In fact the air can go in through the screw part of the piston anyway, and are supposed to be so, if not how can the piston freely move up and down. The sealing part should be at the end of the piston where it meets the ink.
So let me phrase this question, what's really the issue here. IS the converter refusing to let ink in or is it the fit is so loose that it simply fail to keep the piston screw mechanism staying put. If its the former, its probably having some problem more up front, not there at the back end. if its the latter then you may be able to simply cure it by using some thick silicone paste on the mating surface or simply shellac / glue / resin.
Anyway, without any specifics its hard to tell whats the sizing of the converter. Even Jinhao use different sized converter. Take a look at the throat end of the converter. if its significantly wider than the Jinhao you are likely looking for a 3.2mm inner Diameter converter ( Parker/Aurora standard ). if its similar then you've got to be careful. As there are multiple specification of converter that look basically identical. Hero's standard ( mostly used by other Chinese Manufacturers ) call for an inner dia. of 2.6mm , Wing Sung's old standard call for 2.5mm , International standard is more like 2.4mm ( that's why the old Wing Sung pen could use an international converter but not the other way round ). On rare occasions you will encounter even slimmer throat of 2.0, 2.2, and 2.3mm converter for some specific models but I do not think they apply.
If you can inform us of the specific pen model we might be able to narrow it down further, in the mean time, and decent Chinese pen retailer/dealer likely will be able to furnish you with a Jinhao or Hero converter