no heat ,new thermostat blend door working.

[itsnotreallyamazda]

alright here goes, i bought the truck a few months ago and the heat didnt work. so one day i decide to top it off before going about an hour away on the freeway each way.it was warm out so i didnt even check the heat till later that day. i ended up in a hurry and taking nearly the entire trip wide open, petal to the metal. this trucks temp guage never goes even high in the normal range..ever. sometimes within a few mins of starting it it will drop back down to cold and gradually go back to normal. anyways the heat worked great for a couple of months. i think that running it hard for so long got the coolant flowing good enough to expell all the air.
one day, no heati put a new thermostat on and pulled ot up on ramps to get the front end up to fill the system, it seems like no matter how long i let it run and keep filling it it keeps expelling bubbles. all the hoses are hot and it will occasionally blow heat but it gradually gets cold again. which is why i think the blend door isnt the issue. someone apparently removed the vaccum operated coolant shutoff to the heater core because all i can find is the plugged vaccum line. could this being gone cause me to not have heat, or be keeping the system from burping? or do i just need to get the front end higher and be more patient about filling it. it has become a ritual to burp and fill it.
any input will be greatly appreciated