Probably 10 psi would be plenty enough particularly in the case of an old radiator.
To get that pressure all you really need is a 30ft. length of vertical hose or pipe and some way to keep it vertical, a house, a handy tree or even a flagpole would do! If a ”Tee” was introduced into the hose then the head and therefore the pressure could be adjusted via the domestic supply. Atmospheric pressure will support a water column of about 40ft (12.2 metres) so 30ft should give about 10 psi.
