if its a piston type like forge then revered is the way its ment to be (boost pushing against bottom of the piston)
Incorrect.
As Chris and Beppe have stated, charge pressure should be on the side of the piston. This is part of the design that allows a fast acting DV with low spring pressure that will not leak.
By reversing it and allowing boost to act on the bottom of the piston, you're making it as crap as every other piston type BOV that needs high spring tension to remain closed, which in turn leads to it not opening under light load and compressor stall, eventually resulting in turbo damage. If you don't up the spring tension, then it's likely to bleed off boost, so you lose either way.
In theory, the pressure acting on each side of the valve SHOULD be equal and opposite, so it would remain closed, but due to pressure drops across the intercooler core and then again across the throttle body, the pressure at the charge pipe take off will always be higher than the pressure in the inlet which acts to hold the valve closed, hence it leaks.
