I've posted this before a couple of times - turning the prop backwards does NOT damage the vacuum pump. This is from Parker Hannefin, who manufacture them. I've lost the email the service department sent to me years ago confirming this but everyone's free to send their own enquiry. :-)
Anyway, turning the prop backwards is part of the magneto timing procedure and I've never heard of anyone breaking the vanes by doing that.
The pump killer is oil, solvents or water.