Here's an easier way to do this:
1. Remove the spark plug and the rocker arms from the cylinder you're working on.
2. With the piston somewhere other that TDC, feed about a foot of clean, soft cotton rope onto the cylinder thru the spark plug hole.
3. Rotate the engine with a wrench on the crankshaft, until the wad of rope in the cylinder pushes up against the valves.
4. Do your job on the seals.
5. After the valves are back together, rotate the engine the opposite direction until you can remove the rope.