Lots of possibilities BD. Could be failed piston rings, blown head gasket, sticking valves amongst other things. Previously you said it was recently rebuilt by a shop. You need to begin investigating these different possible conditions, but may also want to reach out to the shop on what if any warranty they offered.
.This is a 1957 235. Not the original 235 from 1958. not been rebuilt leak down test next week
~ BD. "You won't find me in an old folks home" Pioneer Florida Cracker 1958 Apache Stepside 1958 Apache 1/2 ton Panel 1959 Apache 1/2 ton Panel
