I have a Parker 51 Vacumatic, on occasion I am getting a small amount of ink leaking onto it? Is this common or something I need to look into repairing?

Yeah, you'll prolly have to get it fixed, sounds like the diaphragm is going. It'll completely fail at some point in the not too distant future.

Without a doubt the diaphragm is shot and needs to be replaced. Ink should not contact the plunger in any way on a Vacumatic fill pen which is working correctly.

Cheers,
Sean

I third the belief that there has to be a tear in the diaphragm somewhere...