Pressure line not filling
I am not a washer repairman. But I have repaired a few & had a run in with a similar problem some years ago. First off, the top of the line has to be vented, otherwise it will pressure lock as it start to fill, ( As your description indicates). I do not know where that vent is located but, you should be able to put a small amount of compressed air on the bottom of the line without damaging anything. The other possible that comes to mind, the samll plastic box at the bottom of the line, ( Assuming there is one on you unit), is, as I remember, a sealed unit. But is it possible that there is a partial obstruction, ( Lint, or whatever), inside there that is metering the flow? If so I would just buy a new unit, they can't be very expensive. Hope that helps. Let me know what you find please.