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Q: Where is the vacuum bypass valve for the heater ? heater core gets hot,no heat. on 2000 Chevrolet 3500 Pickup

2000 Chev 1 ton truck dulley 3500 HD 7.4 Vorteck. I have changed the radiator cap, the thermostat, checked fluid leavel, the temp controls work, the heater core gets hot the hosess in and out are hot, one hose is hoter then the other one ? bought the bypass valve but, can not find it any where its like the truck did not come with one, its not inside or anywhere from the firewall. Bought a Chilton repair manual 1999-06 book #28625 but has nothing about a heater bypass valve The parts store says the vehicle calls for one but, looked it up on the mitchell repair program and could not find anything on the location of the bypass valve, or that it has one, and sugested that someone might have removed it from the vehicle, everything i have looked at or remove that has to do with the heatercore,hoses, ect is all OEM parts and has not been messed with from what i can tell, the only thing i do see being a problem is that their has been an attive put in the radiator/a stop leak product that i thought would clog up things. Qusetion ? will the heatercore work if its cloged up and or the bypass valve ? Do i need to install a bypass valve if their is not one ? The AC dose work and blows very cold air when i use it. Note: heater stoped working 2 weeks after moving to OR from CA drove about 800 miles from CA to OR befor it stoped working, i get just enough warm air to take the chill off the windshield and thats it, the vehicle has 122 K miles on it for a 2000 model and is all OEM factory Please help !!!!!!! Thank you so very much.
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