1959 house with radiant heat plumbed with metal pipes through-out two level home. Do not want to replace but worried about leaks. Anybody have idea's how to locate (and fix if necessary) leaks and ideas to extend the life without replacing with HVAC or new floor on top of existing?

Are you worried about leaks because you have some or are worried you MIGHT get some. You can use ultrasonic detectors and sometimes a very short life radioactive source

You will definitely find out about leaks. Once in concrete, there is little you can do about it but to "play" the waiting game. There are companies that locate leaks with ultrasound. Look them up in your phone book. Back to you. Milo

I've heard about a polymer liquid that can be pumped through systems. But do a lot of research and then some more.

Get references also, let me know how you make out. Good Luck.