I used to have an RF-45 (King model from Taiwan late 90's). Thankfully, my sight glass did not leak but it was a common problem on many machines. You might want to do an internet search because I recall people buying stock replacements only to have the same issue. Or realizing the issue all along was fixed once they did a better installation than factory. Most of the problems stem from the casting hole, oversize or at least not consistent with the glass. Or wishful thinking sealant grout job. Or the sealant itself degrading over time. If the glass to hole is a sloppy fit, you might be better off machining (or maybe 3D printing) a better new flange which bonds the existing glass but more importantly a wider flange to the machine casting face, rather than grouting the edge of the hole which is a bad design IMO. Different RF's seem to have done this a bit different depending on the clone. Obviously clean everything well & use a proper sealant. These machines are also notorious for so-so Bondo / paint jobs & I could see that also contributing to leaks.