Radiator Water Capacity

Unknown

Hi Super 6 Gurus
Would anyone know what the water capacity should be of the radiator of a 1922 Super 6, (i.e. not including the block) ?
Mine is about 1.2 US gallons including the header tank. 
Could the capacity be a fair indication of the efficiency the radiator ?
Presumably one that is furred up would have a smaller capacity than the original specification
I don't seem to be able to stop overheating under extreme conditions ( top gear up a long hill for example). I've tried most of the usual remedies, but am loath to replace the radiator core unless it is absolutely necessary.
Is it possible the waterpump is worn out inside ?
How does one test a water pump which is still attached to the engine ?
Any suggestions would be most helpfull, Robert