Not trying to be a smartass in anyway, but isn't grade 8 hardened steel? So under pressure wouldnt it just snap? Or like you said shear? Try a non hardened type of hardware, as this way it will stretch instead of shear and youwon't need to replace them so often? Just my 2 cents.... good luck!
FYI, grade 8 is hardened and will shear before it really bends much, but it will shear at a much higher stress than say a grade 5 or 3 bolt.
If you downgrade the bolt, it will just fail sooner.