My 1999 Chevy blazer 4x4 4.3L has had no heat since the past month. I flushed my heating lines coming from intake to firewall. I flushed my radiator and reservoir after doing so my heat came back but only for that day. Now I'm not sure if my vacuum lines under the hood from intake to firewall are in the right position? I've been looking for a diagram for my vacuum lines but can't find one. I really wish I could talk to someone that knows what there talking about. I have no idea what could of caused this but I do know my heat started fading over time

 

Are the hoses going to and coming from your heater core hot? Have you checked to see if your thermostat is stuck open? Have you burped your cooling system to remove air since you did the flushing of your core and radiator? Sounds to me like you have air in your cooling system.