I adjusted my Greddy blow of valve a few days back, And seems I have lost a fair amount of boost.
I had to increase the duty cycle a heap to get it back to where it should be. Is this common? Or should i be looking for a replacement! Seems to be a fine line between having it open and no leak boost!

 

You should look for a replacment - the OEM item, works best on these

 

HKS SSVQ (Genuine not ebay copy) won't leak due to it's design and does not need adjusting. Or as has been said go standard. They should not leak.

 

If you buy one just buy from a reputable dealer and buy a quality one to start with,most problem people have with leaking BOV are because they are running a cheap eBay one or a snide copy.SJ.