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I think whoever installed it our way to much preload on it and the spring has gone weak causing the watergate to open and not hold boost, I'll order -34, set that correctly and fit my new electronic boost controller.
I don't think you'll have the same problem if you are fitting a boost controller, you could either use a -34 or -31, the boost controllers solenoid should bleed sufficiently off for either to be used without excessive preloads.