Hot water. Not boiling but very hot, in a cup, take out to garage. Chuck rubber part in for a minute or two, until it softens, fit while still warm.
I use the same method for rubber and plastic fuel / oil / breather lines that are a tight fit, works every time. With say a breather pipe you only have to put the end of the pipe in the hot water. When it cools the rubber re hardens and will be as snug as a snug thing.