Excellent post by my friend @halocline But before you start tearing into the main body of the regulator at least try this one little thing. Mark the seat carrier and regulator body so you can match them up perfectly when reinstalling. Remove the seat and carrier as a unit (don't remove the seat from the carrier) and clean both the piston knife edge and the surface of the seat. Not with a Q-tip as they leave fibers-something like eyeglass gleaners and alcohol. Under good lighting and magnification ensure no fluff or any visible defects are on either the hard or soft seat. Reassemble and try again. This will only take a few minutes and a few psi and may save you a lot of effort.
Edited for (an attempt at) clarity.
Edited for (an attempt at) clarity.
