Engine: SBC, 327CI built in 2003 by professional and reputable builder UPDATED 9/13/19: Redid the leakdown, confirmed no leak through exhaust or intake valves. I've got the valve cover off so detecting air flow into crank was a little more difficult because there are so many places for it to escape, but I'm confident the air is passing the piston rings. Now my question is, what could I have done that would have caused the rings on the one cylinder to go bad? Engine has 25-30k miles on it, compression was great on that cylinder before heads were redone, engine sat for about 1 month while heads were out to machine shop. Backstory: I noticed exhaust on even bank sounded off. Compression tested all, all about 160-165psi except #8 was 0psi. Found #8 exhaust valve to be bent. Took heads into original builder and he discovered springs had lost rate and allowed valve to hang. Replaced all valves, springs, resurfaced, etc. I reinstalled the heads and it runs great. However, I noticed that after heavy deceleration my next use of throttle would send very light cloud of smoke out driver side exhaust. (Exhaust is true dual, no h or x pipe.) Ran compression test again and now #5 cylinder is only building 35-40psi and spark plug is dark. Today, warmed up engine, removed all spark plugs, removed rockers from #5 and did leakdown with regulator set to 100psi. I got it as close to BDC as I could but still expected the crank to turn a little bit. There was no movement and leakdown showed about 96% loss. It did not even seem to pressurize. Ive got no bubbles in coolant and don't seem to detect any air through carburetor or tailpipe. Thought I'd run a test on #1 for contrast and it moved the crank, fully pressurized and showed 1% loss, perfect. Again, pre-head removal/replacement the driver side was great, 160-165psi. Ideas?