The pressure discs will let go at 1,350 psi, but you would have to have it real warm to have it blow. An automotive store can get it, but if carefull enough, you should never need one. Take off the side blow dispurse fitting, and the disc is in there. 1-2-3 and it is done, if you ever have one blow.
I put my bottle in the snow bank, or the fridge at home, and then I warm the 10 lbs. bottle with warm (90°) water, and I use a short brake line with the 4AN fitting on both ends. About 2 1/2 ft. hose is all you need. Listen for the pressure to stop, and you are done. Find a digital scale to use, and experiment. Set the scale to zero, and then turn the valves, and see if the right amount is filling the bottle. This way you are sure. They have small digital scales for under $30 on ebay.