1998 GMC Truck heater controll valve

There should be a water control valve in one of the heater hose lines with either a cable to it, or a vacuum line. See if you can move the valve by hand. If it moves freely, either your cable is bad, or you have no vac, or vac at the wrong time. If you have a vac line, pull it off and see if the valve moves, also see if you have vac. Move the temp control lever and see if you get vac in either hot or cold position. Hope I helped you out.................mickey-d

flushed the heater core and that was it. flushed it backwards. before there was no, i mean NO flow comming out with a H2O hose from house hooked up to outlet, and inlet opened to the air...just used water!!! WOW what came out was black and dirty! thanks.....pat