Any kind of vacuum will work but I do buy the small rolls of Greenlee blow line string.
When you suck the string through the conduit, the string comes out of the center of the roll. You don't have to worry about spooling. You can just set the string on the floor next to the pipe and watch it fly!
By the way, I have better luck with plastic baggies than pistons (or mice).
Interestingly enough, I've had better luck with good ol' Walmart bags o' plenty... Up until I had tried to convince my new partner in crime of such insight, and lo and behold, the bag wouldn't go after three tries and on the first try with a 2" mouse, it came through slick as a whistle. Drat!
Fate always has a tendency to make be look like a fool in front of him... even with my darnedest efforts to do otherwise...
-Virgil Residential/Commercial Inspector 5 Star Inspections Member IAEI
Yea, the light weight bags work the best. It seems that I alway have at least one Lowe's bag in the truck.
The way I do it is I rip off part of the bag and twirl it around but being careful not to twist it too tight. When ever I feel that its just the right size. (Fits loosly in the pipe but touches on all sides when its folded in half) I tie the string on at the fold which will become the leading side when its sucked through the conduit. Stuff it into the pipe, turn the vacuum on and let her rip! When it goes into the vac the vac makes a different noise.
The baggies work especilly better when sucking through field bends in EMT or PVC. The pistons usually get hung up on field bends.
I agree with using as light a plastic as possible. I am going to try to expain my best method. It works great:
Take the plastic bag by the corner, cut the plastic so you have what I call " a little ghost" in your hand. Now take your twine and feed it up into the ghost and poke a hole at the top just big enough for the twine. Pull it through about 2-3 inches and then wrap back around top of the ghost anf tie a knot. I put a little piece of black tape over twine and know. Feed the ghost in the conduit by reversing it so the vacuum actually causes the ghost to seal. Careful cause that sucker "flies" through the pipe. Man that it hard to explain.