Model A radiators when filled to capacity will overflow until they seek an appropriate fluid level. Most drivers find this level covers the top of the tubes but does not fill the tank to full capacity. Continuing to top-off fluid level to the filler neck level will likely result in continued overflow. A clogged radiator may restrict flow and may cause fluid to rise in the tank before overflowing, traveling at higher speeds or even a water pump with excessive flow rate will have the same result.
In all cases when the fluid level rises in the tank, finds the overflow line and evacuates the system you lose fluid and you can potentially overheat.
To prevent this Ford released a Service Bulletin in April 1930 detailing the addition of a baffle in the radiator to “knock down” the fluid and prevent cooling system fluid loss and forward orientation of the overflow line.
Discount radiators may not use a baffle or they may become loose and separate. Clogged radiator can force fluid out and the flow rates of the new leakless water pump impeller may send too much fluid for the radiator tubes to pass if the impeller veins are not “ground” to reduce the flow rate per the instructi0ns. In ANY of these circumstances fluid may be lost and overheating may occur.
Brassworks Model A radiators use a baffle for the same purpose.