My 89 Maxima has just had a complete routine maintenance( thermostat, water pump, ttiming belt , all new hoses , and gaskets) but now the heater doesnt put out like it should. I have noticed that when I take off I can hear water sloshing around . What is the proper way to burp the heater core in order to free it(heater core) of air pockets

 

yes, raise the nose of the car up like 3 ft in the air. then fill the system until no more bubbles comeout.
also make sure the car is at operating temp when doing this.
No I'm not kidding.