I know this a bit of an old thread, but have recently encountered very similar issues so thought I'd share. May help someone. I'm now running oil fed bearings but was having problems with new plastic end caps cracking and spitting oil out everywhere. I found when putting caps on they were really tight so i was tapping them on with a hide hammer, which I think over stressed them. Got a new set and sat them in some boiled water to soften them up for a few minutes. They then pop on easily and I've had none split since. Probably jinxed myself now.
