What Mike said. I usually just put 2 hose clamps on the junction. The fuel injection hose clamps work better than the plain Jane band clamps. If you have worries about slippage, use a double flare adapter on the open end of your access valve tube. Usually it is a two step process, where you use the adapter to bend the tube first, then flare the end back on itself with the "regular" flare die.
To make a "double bubble" on the end, the adapter is put over the tube sticking out of the vice and run down a little bit. The adapter is also used as a depth Gage to set the tubing stickout height from the top of the tubing vice. That's it. Looks like this when it's done:
It's the middle one.
Works like magic.