Mine was leaking about a year ago and I just replaced the rubber o-ring that fits around the sight glass. The plastic glass wasn't cracked, so the new o-ring took care of the problem.
I bought a large assortment of O-rings at Harbor Freight for a couple of bucks.
hotjag1
2003 40' Allegro Bus, 3 slides, 400hp 8.9 liter ISL Cummins
2000 24' Dynamax Isata