Just curious does the service air glad hand have pressure on it all the time or is it just with the brake depressed? How is the emergency glad hand designed to be used?
The Emergency connection has air pressure at all times and the Service connection only when the brakes are applied. Remember, both of them still have an in-line valve you have to ensure is open/closed as needed. (I.E: you don't want to drive around with the Emergency valve open if you're not towing anything - it's a good way to overwork your air compressor or not have the air you need when you need it).
As quoted from here:
http://enginemechanics.tpub.com/14081/css/Trailer-Air-Lines-167.htm
SERVICE AIR LINES.— The service line carries air that is controlled by the foot brake or the trailer hand brake. Depending on how hard the foot brake is engaged, the pressure in the service line will similarly change. The service line is connected to relay valve(s) on the trailer to apply more or less pressure to the trailer brakes. As pressure increases in the service line, the relay valve opens and sends air pressure from the trailer air tanks to the trailer brake chambers, thus applying the trailer brakes.
EMERGENCY AIR LINES.— The emergency line has two purposes. First, it supplies air to the trailer air tanks. Second, the emergency line controls the emergency brakes on the combination vehicle. Loss of air pressure in the emergency line causes the trailer emergency brakes to activate. The pressure loss could be caused by a trailer breaking loose and tearing apart the emergency air hose. The loss could also be the result of a hose, metal tubing, or other parts breaking and causing an air leak. When the emergency line loses pressure, it also causes the tractor protection valve to close, causing the air supply knob to pop out. Emergency lines such as hoses couplers, and other parts, have a red covering. The red covering allows you to separate the emergency lines from the service lines which have a blue covering.
On my M35A2 the handle for the valve is counter intuitive. If it's parallel to the pipe, it's closed, if it's perpendicular, it's open.