2013 Kona Jake, 2008 Kona Major Jake, 2013 Kona Jake the Snake, 1999 Kona Muni Mula, 2012 Ridley Excalibur, 2008 Surly Long Haul Trucker
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.