I've just found the problem with mine, Volvo in their infite wisdome fited an NRV on both the front and rear washer lines close to the pumps on mine. I'd already replaced one a couple of years back (for the rear) as that wasn't flowing so didn't suspect that one was now the cause of that not working properly.
However, because the screenwash i added a few months back turned to something snotty and blocked a few things, it turned out it had turned that new NRV to snot inside as well. Drilled it out and refitted, rear wash action restored!
The fronts had gone weak too, I replaced the T-valve on the underside of the bonnet a couple of years back as the old one was leaking so again, didn't suspect that.
Before i tried in though, i checked the NRV originally fitted by Volvo near the pump. Had to give that the same treatment as the aftermarket one and then got a slight improvement. Had a normal T-piece for a vacuum take-off i added and no longer use so swapped that with the T-valve. Suddenly my screen wash was way better than it ever has been, even when i first fitted the fan-mist jets (from an S/V40, straight fit and vast improvement on the screen wash!) they now squirt higher and harder onto the screen.
Refilled the washer reservoir (again!) and hopefully that's me sorted for a good while!
