Although I do not have any drawings for such a device, they should be readily available on the internet, though it may take a bit of searching to find. These check valves are basically a one-way device used to allow the passage of fluids in only one direction. They get the nickname 'clack valve' from the sound that they make when activated, an audible 'clack' sound. Also they sometimes have been known to oscillate open and closed rapidly during their operation, thus again producing an audible, very annoying and distinct sound. You will require one in your water supply line OS in order to prevent the hydraulic conditions you described. You may have to install an additional bung in the boiler shell to accommodate this line if you do not have a spare fitting available.
BC1
Jim