OK, after fitting larger diameter (internally) pipework i now get -0.7-0.9 bar, but still 0.9 bar on full boost (std WRX Z4 ECU). I used to get a reading of only -0.5 bar on tickover.
Can upping the internal diameter of the pipework from manifold etc change the negative pressure on tickover ? Is this going to cause damage ?
I'm 99% sure the original pipework was 4mm inside, now using 6mm which fits as its flexihose.

 

Are you sure the larger diameter pipe is sealing properly around the connections..?

 

Yup, And i would assume if they were leaking in anyway then the positive pressure would also be affected.