Vacuum Reservoir Plumbing Help Needed

Would anyone be able to give me a detailed, step-by-step procedure in troubleshooting the vacuum reservoir,the control valve,and the vacuum hoses between each of them to the heater/AC controls in the dash?It is exactly as shown in the OP's photos.Hopefully,without removing anything from the dash to get access to the controls?
My son just bought a beautiful 73 Valiant.The heater was blowing cold air.Though I know nothing about the new car or it's heating/cooling system,I recognized that there was no movement in the arm of the control valve under the hood when he messed with the controls.I manually changed the position of the control valve arm,and got heat.When we returned home,I shut the heater off and turned the car off.Twenty minutes later, when I checked,the position of that arm on the control valve had reverted to the original closed position.Can anyone walk me through this?Sorry for the thread hi-jack.