I just worked on a friend's 2002 Subaru Forester that had inoperative windshield washers. The nozzles on the hood were clogged, but after replacing both of those, only one worked. After a little more investigation, I discovered that Subaru, at least back then, inexplicably included one-way check valves in the washer fluid lines just before the nozzles themselves. I'm sure this seemed like a great idea back in 2002, but 16 years later, when one of these values had evolved to a state of not letting washer fluid pass through in either direction, I really have to ask myself what on earth they were thinking.
I can just picture the Subaru engineer in the early 2000s saying "Hey guys, I know that washer fluid systems have been working great for several decades, but maybe we should add a one-way check valve in the lines, just to prevent fluid from somehow draining back into the washer fluid tank, even though that's never happened at any time in the history of washer fluid systems."
I was really wondering why those little plastic fluid line couplers had little directional arrows molded into the side of them.