Had this problem on my '38 about a year ago. Reservoir full, fuse ok, spray had been getting down to just a small dribble, then it blocked up. Being keen on DIY I managed to work out how to get the spray nozzle item out, and took the pipe off. Clearly the problem was the nozzle had blocked up. In the old days you just poked a pin through the rotatable guide-sphere & aimed it where you wanted it. These days it's a one-piece (seemingly!) moulded plastic item, so I got out my tiny drill bits <1mm dia & started operations. Nurse - Dremel! Drilling it out from below felt very spongey! Something odd happening up inside.
Having drilled it out from above & below & re-assembled, it worked a few more days & blocked again. Car was due a service, so I reported both nozzles blocked & they got swapped free under w'ty.
I believe these incorporate a one-way valve, designed to stop the nozzles draining back into the tank under gravity. So you're guaranteed instant water the moment you ask for it, rather than waiting 2 seconds for the pump to prime itself & fill all the pipework as in the olden days. And I reckon it's this one-way valve, perhaps made of extra-flexible plastic, that's breaking down & gumming up the entire thing form the inside.
Next time, when car's out of w'ty, I'll buy a set from the dealer & swap them myself. New set's been fine, so 3 more years life left in them I hope...
