50 yr old NY home. Have had 2 of my wallboard radiators develop leaks so far this winter. Have found the pipes going to/from manifolds to be steel. Have had several HD "Master Plumbers" tell me I'm nuts...yeah okay I put a magnet to them and can see, touch, feel...Anyways, they appear to be soldered at the fittings on the ends, but (again) was told "you can't solder steel, therefore they have to be copper". What is the best way to fix. I am considering going to all pex (but $ at his point very tight). Also this is all 1/2 " lines, there are no bleeders on the wall units, any help?