I agree with John, cracked tube end is the first thing to look for. The other is an egg shaped tube - usually done when someone jerks the tube back and forth to get it out - then the o ring is tight at two spots and loose at two spots - like tight at 12 and 6 and loose at 3 and 9 O'Clock. Brake fluid swells the loose spots. We never used any sealer in production - new tube and new seal. No issue until the seal cooked from temperature or age. The sleeve fixed the tube issue as it rode up higher on the tube and no o ring groove plus no egg shapping.