If you follow the hoses from the valves, you'll find that there is a valve at each end of each line. Two lines going to the heater cores. One for the front heat (cab) and one for the sleeper. I could be wrong, but I believe that they were actually put there so that when there is any work done on control valves or replacing cores, you can shut off the water while you break the connections and don't have 12 gallons of coolant running all over. They do make it better in summer though. The control valves restrict or open the flow, but do not shut it off completely. When it's 90 degrees out, you don't need any heat, so shut them off and enjoy the full AC.

 

Yes, you are exactly right. I followed them are they are exactly like you said. I can see myself turning them on and off depending where I am during the year. Sounds like a pain. I may look into some electronic valve.

Thanks!