Mitsubishi Pajero radiator back pressure

Hi, I am having a problem with air bubbles inside my radiator (backpressure), actually, I have brought it for a checkup at my local mitsubishi dealer and they advised me that the cylinder head need to replace which i did, basically since i have to replace the head and the car is about 9 years old, i thought it would be better that i might as well replace the piston rings and head gasket as well, by the way, i had also taken out the thermostat control of the car since i stay in tropical climate and that is what we normally do here in the Phipppines because we really dont need it. Anyway, when everything was put back together, we started the car and the problem persist, the problem is that there are still bubbles comming out of the radiator and the other thing is that overflowed water from the radiator that goes to the reserve tank does not go back to the radiator....is there something i missed? (sorry, i forgot to state that i own a mitsubishi pajer 2x4 fieldmaster, 2001 model) Thanks