You may not like my advice here, because I'm sure you've sunk some serious hours into your design, but just buy an off the shelf pressure relief valve and adapt it to your tubing.

Real engineers (not saying you aren't one) have spent time getting paid a lot of money and running hundreds of hours of tests to develop good spring-loaded pressure relief valves that work dependably and cost not very much money. Try not to get bogged down with reinventing the tangential details of your project, you'll just sink days into something that only barely applies to what you originally set out to do.