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Very nice idea. Such a simple design I don't even need to read the instructions.I may need to buy a larger bench vice however!Perhaps 2 vices in parallel to make a very wide brake, and to allow a bend further inside the material. Using a single vise limits the depth of a bend from the edge of the material to the distance from the top of the jaws to the bottom of the vice.
As an alternative to adding more pipe to reduce your air pressure you might try adding a "T" to the straight pipe. You could then cap the T or not to change air pressure. This would keep the whole set-up a bit more compact. With a threaded cap and a split at the end of the "T" pipe you could gain some adjustment capability. I suggest this presuming that the air pump you have has only 1 speed.