Having lunched a C10 after trying to start it one morning. Not even a full crank rev and BANG! The culprit was a cylinder full of fuel. The cause was a mix of "user error" and a less than effective tank valve. This is/was a known problem. Although engine vacuum was supposed to get fuel flowing, the petcock didn't always close. Which mean fuel drained into the motor, found its way into a cylinder. Hitting the starter and trying to compressible fluid had obvious consequences,
User error = trusting the "automatic" petcock valve.
How fuel drained into a cylinder escapes me, but it did. As I said, C10's were known to do this.. That is, this wasn't an isolated event.
So - that the motor wouldn't crank if there was fuel in #2 is no surprise. What does surprise me is a cylinder full of fuel didn't bend a rod or worse. And, of course, how did the cylinder fill while the motor ran? Possibly/probably the cylinder wasn't full, and maybe a valve was open, avoiding hydraulic death. But then, why was it there after returning home? Passing the leakdown test explains why fuel didn't trickle into the oil (too bad no one sniffed the oil and strained the some oil through a coffee filter.)
Not to rain on the parade but... I won't be surprised to hear the motor eventually crunched. With no explanation for the mystery of fuel accumulated where it shoudn't, I wouldn't plan on riding across Death Valley.
