If it can't hold 10 psi, you will have "real" ale. Real ales are conditioned without pressure, not a bad thing. I like low fizz ales myself. So long as nothing enters the cask, your ale will be safe.
FYI - A cornie/Cornelius keg is a 5 gallon (US) stainless steel can orignally used for soda/soft drinks. Their relief valves pop at around 135 PSI.
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