Just posting to say that I have replaced the piston in my diverter valve and serviced it with new o-rings and it has completely transformed the car.
When I opened it up the, o-ring at the top which pushes against the screw top had a slight split in it so was dropping the piston a bit too much during the pen test.
The car has never really run that smoothly, minor fluctuations on idle and never providing smooth constant speed when on boost. I had checked all pipes, coilpacks, IAT, MAF, plugs etc even started replacing crank and knock sensors. I checked the spring in the valve which is still strong and did the pen test a long time ago but never thought to check if boost was passing through between the two larger ports.
This change fixed it and car is much smoother now.
p.s. This might not fix your issues but fully test your valve if you're having boost leak (non-smooth running) related issues
