It seems like the rubber gaskets in those caps get all out of shape. If you fiddle around with the rubber seals, you might get it to work better.
2001 RMK 800 151"
1990 Indy 400 (Slightly modded with welded tunnel cracks and various used Indy parts from models throughout the 90's)
2006 Stihl Chainsaw
1980's Vintage Lawnboy Lawnmower (3 in collection)
2005 Husqvarna Weed Whacker (It sucks to rewind the string, but that 31cc motor sure is torquey!)
2000 Polaris XCSP Edge 600 (1.5" Track, 22 40 gearing)