Dripping MagnaClean problem

[Mike13]

Had an extension put on so needed a new boiler and Rads, MagnaClean has been great, but just recently it has been dripping from the top cap.

Do I just need some decent silicone grease, and how tight do I need to do the top cap?
Or do I need to replace the O-ring?

 

[Mike13]

Solved, last time I cleaned it out I had not completely cleared all the sludge on the O-ring. It was feeling rough where the sludge stayed put. Gave it a good clean, slapped on some MS4 and it seems to be holding this time.