You gotta love that drill press lathe piston I made,I got the tolerances down to the inth. Those guides are a snug fit in the pump tube and the brass never touches the inside of the pump tube. I have some pictures of my set up here,you won't believe it but it works amazingly well.
I also have a fixture that mounts in my drill press that allows me to tap the hole in the piston perfectly straight.My chuck only opens to 1/2" and the piston is nearly 5/8" so I have to tap the 5/16" hole just to fit the piston in the drill press.The trick is to make sure I get the hole drilled dead center.
The dremel and drill press both run at the same time for this to work. I do have three years of occupational machining experiance on a lathe and milling machine but they never taught me this trick.