Either use a borescope to check for a scored cylinder, or pull the head and oil pan and remove the piston from the low compression cylinder. You're probably dealing with a ring that was broken during the installation process. Was the engine bored during the "rebuild", or did it just get a new set of rings? A badly tapered cylinder can cause rings to lose their tension and compression sealing ability very quickly. There's a procedure in the tech tips section for checking cylinder taper with a piston ring, a feeler gauge, and a little work with a calculator. If the taper is much over .005" the rings aren't going to last very long.
Jerry
