940 B230FT no boost

[niloc]

I've replaced a head gasket on a 940 T, and now it's not boosting.
Accelerating with the gauge in vacuum, it acts normally, but when I throttle up the turbo gauge goes to slightly above atmospheric then drops down to about 11 o'clock position. This happens at about 2.5k revs. When the gauge drops down, I get very little acceleration regardless how much throttle I give.
I noticed the diaphragm in the bypass valve had a hole in it, so I replaced it with a known good one from a spare turbo I had. This made no difference.
Any ideas guys?