I remembered once I had the same problem with my Underground service truck , it would squirt water for a little while then stop , and like you said not work for a few days. No real pattern. It did my head in as the saltwater from the mine was constantly spurting onto the windscreen it had to be fixed.(saftey issue).
I inspected , modified , adjusted ,theorized ect ect.
This was a way shorter circuit than your volvo's but I replaced the check valve as you said you have done ( no change ) Blew out the entire length of the fluid line ( no change ) then I replaced the whole fluid line pipe and it worked fine. I am pretty sure that where the fluid line passed under the trim near the passengers feet area was the place over time where people constantly treading on the trim slowly squashed the pipe restricting the flow . Air seemed to pass through but if the diameter is reduced in size I would think that this is where the water has a problem getting through.
( My experience , theory and remedy )

Si850