tony_g wrote:
....w/ tale of woe subject...

I've not followed this thread, but a comment about grit/sand/etc.,
wearing seat caught my eye---methinks it's quite likely the seat is worn
from erosion from the previous leaking (a very small drip still comes
through the minute area between the seat and washer w/ pretty high
velocity/pressure) and the problem is a new washer can fill the
imperfection for a short time but soon fails.

If possible to reach, you could possibly clean up the seat face s/ a
seat-facing tool if that's the case.