Hm, ok, so I am likely looking at a new compressor. Yes, the conversion could be sketchy, there are no stickers indicating it has been done...
I have definitely not overfilled it, as it was nearly empty and I only put one can in. I do have a set of gauges and even when the compressor was working at first the high side never went much above 100 psi.
Thank for the info!

I will be tackling the dragging rear brake this afternoon after our Fiat club's lunch meeting :-)

Csaba