2013 Kona Jake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 2006 Kona Kula, 2012 Ridley Excalibur, 2008 Surly Long Haul Trucker, 2001 LeMond Buenos Aires
I've been using TRP HY/RD's for about five months. I had some problems when the weather turned cold, but nothing like that. My issue was just low fluid levels in the hydraulic chamber. After I topped it off, they've been great down to 20F (the coldest I've had occasion to try them).
On the other hand, I noticed Barry Wicks was using HY/RD's at the Cross Crusade race in Bend in October, but in a picture from this weekend's Deschutes Brewery Cup I saw BB7's on his bike, so who knows what might happen as you approach zero Fahrenheit.