These kegs do hold pressure. I've already cleaned and reconditioned them with O-rings and poppets, and replaced lids on three of them with used Cornelius ones since replacement pressure relief valves on the old Firestone lids were impossible to find. The kegs were $18 but were really nasty when I got them.
New liquid posts for these kegs are about $8 each I think. Not much really, but almost half of what I paid for the whole keg before replacing other parts.
I think I'll try the "boil the hell out of it" suggestion and hope for the best.