I'm sorry, dont take this as me burning you. But the R33 GTST BOV is almost and quite literally is the 1G DSM BOV common to honda boosters. They do have a leak point and though they are vacume driven, they do have a limit, There is a port on the charge side that will OVER-RIDE the Vacume, this is a failsafe to fuel cut and overboost in the case that the OEM wastegate fails... You can see 14 PSI to 15.5 PSI BUT, there will be leakage. Aftermarket BOVs also have limits, but they can typically be changed with a simple spring swap... A good trick to bypass that BOV's leak point is 1st, block off that Bypass, and 2nd, this can be sketchy for some people, crush the top of the BOV in a little to compress the spring a little more to increase overall boost hold. I hope this helps.
EDIT: Though the Stock bov has trouble reverting boost pressure at about 15 psi.