Maybe the one way valve is leaking?
I used a similar setup for a few months at one time and it worked perfectly. Although it was not a one way valve, it was simply a T inline with the MAP sensor with one side left to the atmosphere (I know unmetered air etc, but it worked fine).
Another option is to put the 97+ MAP sensor, which is what I'm using now, by default it will cut boost at 1.5bar. It has a different electrical socket, so there is a bit of re-wiring to do.