We originally hydroed one tank to failure. It failed at 495 PSI.
We made the end cap that failed thicker, and assumed we fixed the problem.
Today I asked my son to hydro the test stand to 500 PSI since we had not done so. The first tank in the test stand failed at 475. He then hydro tested the tank we had heat treated. It failed at 425. So to complete the set we tested the other tank on the test stand to failure it failed at 560PSI. All three failed in exactly the same spot in the weld between the end cap and tank end. Its clear that our quality control needs some work. On Tuesday we will hydro all 8 tanks on the actual vehicle and see how that goes. Its really hard to tell if the failure is in the heat effected zone on the margin next to the weld, or if the welds them selves are too thin.