I have a very old gas furnace was build on 1965,it still works very good, but I found out there is a little problem :when the gas valve opens,there has a little explosion before the furnace lit up;there is a small tube came out from gas valve and go to near pilot light (small fire old style not electronics type), the pilot sensor is a metal build on ceramic with 2 screws wire connection series with gas valve (in case no pilot to shut off gas valve), what I guess the tube is to make the smooth the starting of lit up the main fire(furnace on),when the gas valve opens, a little gas flow through the tube go to the lit pilot before too much gas spread around and makes as a big pilot thats smooth the heat goes on( big gas lit up) ,also it service as the pilot if the regular pilot extinguished during heat was on, I found the tube has no gas came out if my guess the tube was for,anybody can tell me what the tube for ?Am I right? Thanks. George

I believe the tube you are referring to is the old style safety bleed tube. it allows a pressure balance without venting the gas into the air because they burn it off in the combustion chamber.

Thank you for your answer.Another question :is there suppose has certain level of oil in the gas valve chamber ? (when open the top cover of the gas valve:half of the relay dipped inside the likes oil well)

You are not supposed to open the gas valve. There are no used serviceable parts inside. Call for service before you hurt yourself and other.