Service brakes won't dump pressure?

The blue line to the trailer is just a signal line to control the trailer relay valve(s). That line should exhaust through the foot valve or hand valve depending on what you're using. Any ice or blockage in that line/valve exhaust port is going to lock air in the blue line and keep the trailer brakes on.

The hand valve shouldn't be dumping directly to exhaust though. Quite possible that valve has failed. May even be a possible cause of your previous trailer brake issue.

 

First thing you need to do when you get home is repair or upgrade your air dryer. In the meantime, keep draining those tanks daily.

 

You will see a short air vent line leading out of air valve next to drive axles that can ice closed and if you move it around the ice may come out?

 

You know I would bring it to any dealer to have it looked at.

For some reason this may be a case that the replacement part is not an exact replacement part, I had a problem like this with a ws and air suspension manifold, the original had the ports on way and the replacement had to two additional ports, the mechanic blocked the new ports and the suspension didn't respond to the pilot air. We finally took it to the dealer and it was fixed in an hour with an explanation of the real problem.

 

If moisture is the problem, the moisture is in the lines and valves, still. Also drain the trailer tanks. You can use various “air system anti-moisture” products but follow directions carefully. Otherwise wait until it finally warms up above freezing for several hours then spend some time cycling the service brakes to help push the moisture through the lines and out.

 

The Traction Control system valves are very sensitive to moisture and especially ice.