Hot Water Issue

We have recently installed a outside hot water boiler.  We brought the lines in and have three zones off of them.  The bottom zone goes around the main floor the top zone is to the upstairs.  We have the old cast iron radiators in the upstairs and baseboard in the down stairs.  Down stairs works perfectly fine all the time.  The second floor is where the problem occurs.  We can get them to fill up when the cirrculator is running and then when the thermostat kicks the cirrucrulator off the water back drains out of the radiators.  We piped them so the each room can be turned down at the radiator.  The way we did this was to run a main line in the ceiling of the first floor and we T'd off to the in and out on each radiator.  The in and out are both on the bottom of the radiators.  Any ideas on how to fix the water leaving the radiators and going back through the cirrculator and out to the stove would be greatly appraited.