CoreyR

try loosening the screw all the way out. that screw is there to put pressure on the internal spring so that way the bov doesnt leak under higher boost levels. its probably just set too tight to blow off at your current level

B16Civic93

if it blows off under high boost and not low boost then that is because like mentioned above the valve is set too tight, you need to loosen the adjustment screw so it opens up easier, but make sure its not too loose that it will leak boost pressure. Why are you feeding the brake booster from the vacuum manifold.. the hose is probably too small if thats the case. you need to have that 1/2inch or 5/8 or whatever size it is coming off the intake manifold and going directly into the brake booster with a full size T that feeds the vacuum manifold separately... you really need to get some pictures up so we can see what you've got going on