So my Shop-Vac is quite strong, much stronger than I really need. In the past I have had troubles with it collapsing my dust collector.  It has always baffled me that they don't have a vacuum relief valve built right into it.  All my home vacuums have one, and they act as a good indicator that something is wrong.

I was contemplating various ways to add a relief valve and finally hit on replacing the drain cap.  That lets me reverse the modification when I need more pressure (ie when sucking up water) and yet it is large enough to allow for good pressure relief.

This is made up of just two 3d printed parts and a m6 bolt and spring.

And it is designed to print without any supports.  Here you can see a cross section of the cap from an early test print.  It fits quite nicely.

This works quite well and I'm very pleased with it.  I have only done some small tests with it, but it seems to provide a good level of protection.  Time will tell if it is truly a good idea, but I'm very happy with it.

You can download the stl files from my Thingiverse page.