Hi David,
I guess I know the Smooth Stop secrets better than anyone since I designed the thing.

Some time ago the supplier of the brass tube that forms the cylinder changed the spec on the tubing without telling their customers. It would appear that the effect of the change left the tube with a larger ID, so the O-rings don’t seal as they should. It may be that you have a couple of these. BVM updated the design when they found out about this. BVM has always offered a free repair for this, which I know is a bigger pain for your location.

These can be fixed, but it is a pain, I did one for a friend once. The only way I could figure out to fix it was to wrap some fine copper wire in the bottom of the glands and add little silver solder. Then, using a lathe, re-cut the glands so that the O-rings are compressed very slightly when inserted back into the brass tube cylinder. Unfortunately the fix requires changing things by a few thousands of an inch, so just changing something out probably will not help. I am sure that there are other ways to approach this. Of course this might not be the cause of your problem.
Steven