Heater Issues don't know what to do

[loganfireman]

I have a 1999 F150 2wd 4.6 engine. I have had a small coolant leak in the heater core for some time. but it always blew hot air with a little smell of anti freeze. Last week it started blowing cold air once the truck got to operating temp. Then 2 days later the heater core started to leak really bad blowing anti freeze on the wind shield. I followed the instructions on here. (very good instructions by the way) and replaced the heater core. Also replaced heater hoses and flushed the coolant system. After replacing the heater core I had no leak but the same issue of warm air until the truck got to operating temp then only cold air was still there. After ready I decided to replace the water pump and thermostat. With 250,000 miles plus I did not think it could hurt. Flushed the coolant system again. But had the same problem. Talked to a local mechanic and he said it sounded like I had air in the system so I took the radiator cap off and ran the truck for about 20 minutes until the air bubbled out of the system. I refilled the system and ran another 20 minutes but no more air bubbles. I checked and still had the same problem hot air until the truck got up to temp then cold air. I checked the heater hoses and both the entry and exit lines are hot. I feel that it must be some type of vacuum leak but I do not know where to start. Does anyone have any suggestion? I am at a road block and do not know which way to go. Any help would be appreciated. I also want to say thank you for all the previous post on here I have used them many times to fix small issues on my truck.

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[10thGenF150]

If you have a vacuum leak all the air will default to the defrost.

[loganfireman]

Thanks 10thgen I was thinking wrong I guess I was thinking that the blend door and the air divert switch were on different lines. well i am really confused now. The selector between defrost, floor and vent works just fine all the time.

[loganfireman]

Well I think I may be on to something now. Got an email and it explained how the blend door will act up when it is cracked just before breaking completely. So I will be checking it asap. Thanks for all the help.