On Sat, 11 May 2013 01:25:08 -0400, Larry Weil wrote:

I have one of the cans with the vent (it's about 24 years old), but the
cap on the vent has split and I am unable to find a replacement for the
cap. Anyone have a good idea for a substitute?

Check into carb / manifold rubber vacuum caps... They may work for
you.

Sample:

http://www.tdperformance.com/cache/ex_q75_w500_h500_images_ePIM_original_9009.jpg