What is a recommended vacuum line to use for the back nipple of my hks bov

 

4-5mm silicone? I have no idea that size the nipple is on the BOV

 

I would agree that silicone is best, generally. The right diameter, of course. Here's Thumper's post about Z31 vacuum hose replacement. Vacuum Lines If you check the text, he says there can be problems with silicone for certain hose locations. So let us know what you decide to use and how it works!