I changed heater core in car now i have back pressure at the water reservoir,what causes this?

try reversing the hoses to the heater core...|||If there%26#039;s a bleeder valve in the gooseneck, let it get hot, turn on the heat and open the valve, if not, just run it with the cap off till it%26#039;s hot.|||Probally air in the system. Did you bleed it ? What kind of car ? Are you turning the heater on while you fill it with water ?|||Remember, when you fill the system, fill about a gallon, then have the vehicle running, and turn your heater on high, then add the rest slowly with an idle of about 1,500 to 2,000 rpm.|||you have an air lock. when refilling collant,find the highest level of water ,like highest water hose,fill till it wont take anymore ,losen that hose and bleed the air out of the system. then keep filling till water comes out of that hose,reinstall that hose , fill it up .crank let engine run till the themostate opens.shut off engine let coll then ck water level,top it off ,this should help,if not poss head gasket could be blown.GET ER DONE..