Help with cooling!
 

Hi all,
I'm working on a '64 Bedford TK fire engine (petrol) which overheats.
Cool/cold water starts coming out of the header tank fairly soon after starting and continues without getting noticeably warmer. We have checked the thermostat and flushed the system by introducing a hose into the heater pipe and venting from the bottom rad hose.
I can only think the water pump is faulty (no obvious leaks) or rad blocked?
There appears to be some sort of heat exchanger in the system with 2" rad pipe going through it and two unconnected smaller stub pipes coming out the bottom - does anyone know the function of this?
John