i dont know what they call them but its a little inline thing for the vacuum pipes in your car...
i have a few they fit 8-14mm tube and have a cylinder in the middle of the 2 pipe fittings, in which there is a little rubber circle that presses up against a stopper when air (is from vacuum being drawn through) is being sucked toward the vacuum source and allows the flow, however when you reverse the flow (ie suckback) the stopper is not at the other end and is pushed back immediatly and covers the hole at the other end, preventing loss of vacuum...
Oeverall size is 2.5" long (including tube connections) and 1" diameter.
They would not hold under chemical abuse so a catch system to filter anything off is required....
I thought thats what the packet said, but i cant remember accuratly, it was a while ago, you buy them in single blister packs from major auto stores...
-AC