I live in Michigan and in less than a month of winter they have seized my valve stems so much they had to be cut off. It ended up being about $80 to repair ($20 per wheel for a new stock plus reinstallation and balancing). I knew aluminum had corrosion issues with steel in wet weather, but I figured maybe not brass. I planned on replacing them if they ever rusted in place. But it went much faster than I expected. It can be safe if antifreeze or grease is applied to the threads. But for me, aluminum caps just aren't worth the hassle, I've switched back to plastic.