My vent cap/nipple was rusted solid. Couldn't open it up or find a new one anywhere. Made my own out of a 1/8 threaded by 1/4 barb brass fitting. Then crimped closed one end of a 1/4 brass insert bushing for plastic pipe. Left the other end open and made a slight crimp in the bushing body. Put this over the barb fitting. The brass bushing rises and falls over the barb fitting but the small crimp in the body prevents the bushing from rising over the barb end of the fitting. Works fine.
