It was at about 4 psi when it went pop with quit a loud bang as I was just pulling away from a junction in first gear gently. It has split the weld holding the 2 parts together at 90 degrees. This is the second time it's happened at the same point. Last time it was welded up by a specialist welding company so don't think it was the weld at fault. Plus it's not just a cracked weld, it's bent the metal both sides of the weld so was opened up with some pressure.