Even with a pretty big leak, it would be possible to overpressure the cooling system with an air hose, so restrict the airflow somewhat, either with a pressure regulator or by putting a kink into the hose. A quick and dirty flow control could be a zip tie over a pinched-down air hose to let a moderate volume enter the radiator through a hose wrapped with a shop towel inserted into the fill hole in the radiator.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!