I seen a silicone that was not pure it had an additive that made the O ring swell especially for sealing in HP uses. Have you used anything like this.I did try a thicker o-ring with no luck, I think part of the problem is that the o-ring at the base of the plenum is the exact diameter of the valve stem retaining screw and sits right on the groove between the cap and the inner diameter of the valve housing, the spacer that originally came with the factory huma reg is an ever so slightly larger diameter and actually sits on the flat face of the valve housing and not a few millimeters inward where the valve stem retaining cap screws into the valve housing like it does with the XXL plenum.
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