Pilot won't stay lit beyond releasing the button to light it. Replaced the thermocouple--same result. 45 year old Pioneer heater. Tested for power at the transformer--OK. What next?

Is the pilot flame touching the thermocouple, and is the thermocouple positioned properly? Only 1/4"-3/8" of the tip should be in the pilot flame. If the gas control has an E.C.O. device (look for 2 wires sticking out just below the thermocouple connection), it may have failed. Otherwise, the safety coil circuit in the gas control has failed.