To flow back from your top mounted radiators the fluid would have to evacuate the radiator, unless atmospheric air can get there only vacuum can then remain. Vacuum excerts no pressure on the fluid, while atmospheric pressure works on the fluid in the reservoir. Atmospheric pressure is approximately equal to the pressure of a 10m water column, so unless your radiator is mounted more than 10m above your reservoir or atmospheric air can get into the radiator the water will be kept firmly in place ...