Final answer:
When a hissing sound indicating valve blow-by is heard while pulling a propeller through, a cylinder compression test should be performed next by a qualified mechanic to diagnose the issue.
Step-by-step explanation:
If a hissing sound is heard while pulling a propeller through, indicating valve blow-by, the procedure that should be performed next would be to conduct a cylinder compression test. This test helps to determine the integrity of the cylinder and its components, including the valves, piston rings, and the cylinder wall itself. A mechanic should perform this test to diagnose the exact cause of the blow-by.
It is important not to attempt the test without the proper tools and knowledge as it may lead to further engine damage. When this condition is detected, the next procedure should be to inspect and repair the engine's valves and rings to restore proper compression.