luis137 wrote:I used the blank disk to see if the OPV is doing its job and I get to the preset 10.5 bar limit...
In terms of the brew pressure behavior you observe, a blind basket behaves like a 100% "choked" extraction. Since the reading is 10.5 bar, that rules out a problem with the pump or the OPV, leaving only what takes the place of the blind basket: The coffee puck itself.
Is the espresso pouring forth profusely or only a dribble? If the former, the answer is either (a) grind finer, (b) dose more, (c) get fresher coffee, and/or (d) correct distribution to avoid channeling. If you only see a dribble/no flow from the grouphead and the brew pressure doesn't rise to ~10.5 bar as it did for the blind basket, I have no explanation.