1987 C4 heater problem

My 87 does not put out any heat through the heater. It seems the heater control valve is not operating. However if you remove the vaccum tube from the valve, it opens and the heater core heats up. Is this a faulty valve or something else that controls the valve. The valve seems to function by removing the vaccum tube so before I invest in a new valve I'd like to know if that is what is needed.

Thanks........