The cooling system on a Model A Ford can get pretty crudded up with rust, calcium, and also with grease from the water pump. If the radiator has been in service for a long time it is best to remove it and take it to a radiator shop (preferably to one that is knowledgeable of Model A radiators) and have it rodded out.
The engine block and the head are also usually crudded up, especially with rust. This is where a technique called back flushing can be affective. Back flushing of the radiator can also be affective if the cooling tubes are not hopelessly plugged up. The trick is to force water through the system backwards to the direction it normally flows. To read more click here to open the full Back Flush April 2014