The flow of hot coolant through your GMC heater hoses only occurs when the heater control device is either switched to heater or defrost mode. At all other times, the flow is stopped so heat will not be generated that could become a discomfort for the passengers of the GMC truck.
There are two heater hoses on all GMC trucks. One is the supply while the other is the return hose. This makes it possible for the coolant to flow from the engine to the heater core in an efficient manner. The outer appearance of the GMC heater hoses is similar in diameter to a standard garden hose, but its construction is vastly different. The inside diameter is smaller which makes the walls of this reinforced hose more compatible with the rigors of being used to transport high volumes of hot coolant when the heater is in demand.
Because of the confines of the engine compartment with the need to make use of all available space, the heater hoses used on then GMC trucks are preformed so they can follow a particular path from the front of the engine compartment to the fire wall. In the dash behind the firewall is the location of the heater core where the hoses are attached too.
The testing for a leak in the heater hoses is similar to any of the other coolant hoses except that the heater must be activated so the flow of coolant is occurring in the hoses in question. Once the leak is located then replacement should occur. This repair of the leaking heater hose should happen once the engine is cooled down so the technician will not be burned by the hot coolant.