Yes, i do get good heat at the back, they get nice and hot like the front. When i flushed my heater core i ended up buying a new heater valve. The old one still works but was just old so would open and close slowly. I had heat for about a week before going on a run 12hr drive and about 2 hrs into my drive i felt a noticeable change that I would only get heat again while accelerating heavy above 3k rpms, otherwise it turned lukewarm.Are you getting reliable heat at the rear vents ? The rear has it's own heater core, and no control valve to interfere with flow, supplying constant hot coolant to the rear.
Is your hot water valve functioning properly ? ( hot water valve modulates front heat only...) . Have an assistant run the selected temperature up and down while you watch the hot water valve to make sure the valve linkage is moving. Or you can select cold temp on the HVAC panel and pull the vacuum tube off the hot water valve and you should see the arm move to the open position and the heater hose aft of the valve get hot.
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I feel there is no air in my system as I have tried purging it several times and nothing ever changes, however while accelerating to 3500 then back to idle it does heat up alot just to cool back down.