I've used this Gardner-Bender CO2 system and rarely had problems. Slight pressure does have to be maintained on the poly-line or jet line to feel the travel; if there's a blockage, the string can jam inside the raceway and become very difficult to pull back out. The rubber piece on the end of the hose has a small metal tube to feed the string through.
http://www.amazon.com/Gardner-Bender-JO225LC-Conduit-Fishing/dp/B008KKSFUY
http://www.amazon.com/Gardner-Bender-JO225LC-Conduit-Fishing/dp/B008KKSFUY