Vacuum Pumps
Sorry - it's a myth about not turning the engine backwards by hand: it doesn't harm the carbon vanes. The only way you can damage the vanes is to fit a clockwise-rotating pump onto a counter-clockwise drive (CW - CCW) or vice-versa and then run the engine. I've got a letter to this effect from the Parker-Hannefin Service Department.
Props are routinely rotated backwards during maintenance during magneto timing with no adverse effect.
The quickest way to kill a vacuum pump is to let oil/solvents get onto the vanes.