[wolfox]

Conceivably, it would not take much. Hose pressure can be anywhere from 20~90 PSI depending on where you lived. Cooling systems are made to take 15-16 PSI or so. I never stuck a hose in a radiator hose or cap opening to flush because of this fear of "breaking" something I always held.

Radiator flushing is usually done with the radiator removed from the vehicle - well, it was done this way at every radiator shop I ever stopped and looked into. *shrug*