vacuum pump rebuild on 300d advice

I took out the vacuum pump on my 1982 300d turbo. I purchased a rebuild kit for it and I am trying to rebuild it. what I've doen so far: I removed the pump from the car and bolted it to a board with a socket under the lever to release the spring. I then removed the cover and cleaned off the oil and removed the 3 one way valves and the O-rings. I also removed the bolt holding the disc on but I am having some trouble getting the piston off (not sure its correct name). Is there a trick to remove the piston? Are there any other tricks to the rebuild? Thanks in advance.