You stated earlier that it was a 735, not a 745
Hence the confusion. Sounds like the dealer didn't do their homework. Leakdown would have led to checking the leaking component. If the air was leaking into the intake then they would have known it was an intake valve problem, same with exhaust. If the air was leaking into the crankcase then the cylinder or rings would be suspect. It is a way of eliminating the mechanical components of any engine. At least the problem was resolved before the valve broke and the engine was trashed.