Its probably the Ranco heater control valve which is bolted to the heater core. They have not been reproduced. I had the same issue with my 63 about 6 months ago. There is only 1 guy I know of that rebuilds them....Ill find the address.


http://s971.photobucket.com/albums/ae197/jon63c10
1963 c-10 shortbed stepside 427 BBC T16 HD 3 speed
1963 c-10 shortbed fleetside 348 "w" 1st gen big block & powerglide