You can take the valve out & then fab up a plate & gasket to block the mounting hole off, then you could get a true test.
A bleed off at or around the valve wont harm anything far as what the presure test is showing.
Air leaking out around the seals, gaskets or mounting surfaces on the cases will be what you need to look for.
Many years ago, most all 2 stroke repair manuals, youst to show when presure testing a engine you would submerge the engine down in water. Right away any leak would be shown instantly.
Never was to fond of putting my Mccllough 101 kart engine under water. Dunking it wasnt the problem, it was taking it back apart to make sure no water got where it wasnt suppose to that I didnt like.
Neil