2001 Chevy S-10

There are a couple things to do here. You can try jacking up the rear orf the truck and running it whileadding coolant. Or once the truck heats up and the top hose is warm/hot take a pair of channel locks or vise grip an dhold the hose together as tight as you can to fill the heater and get the air lock out of it. You can look for a little screw type fitting on the thermostat housind an dstart the truck and open the fitting to let the air out or you can fill up your plastic coolant tank and drive the turck, but make sure your radiator cap is good. Then after a day or so fill the coolant tank up to the proper level because it should be down by then. Your truck probably won't overheat, and i'm sure your heater doesn't need to work in late July or early August. That should work the air out of the system and your heater will fill.

There is no valve. Best thing to do there if it only trickles out is to disconnect both hoses and the side that trickles hook up a hose (from House) to that side and turn on the water and see if it will backflush. If you have added a head gasket sealer they sometimes clog the heater core.