The windscreen washer tubing under the dash varies, but some cars have a very thin walled black plastic tube which is similar to a cable cover, and very floppy. At each end, to assist holding it in place is a thin ring of the same tube over the top. One of mine was missing.
My tubing has never leaked but watching it, while squirting the screen shows that it spasms quite violently, and could easily detach itself.
This image shows the original black tubes. From the pump to the T piece is a more conventional clear tube. The black leather in the foreground is a cover I added to keep water away from the instruments and circuit board.
This is the original tubing next to the aquarium tubing I sourced from e bay.The size is 3mm. You can see the extra rings at each end on the original tubes.
Here are the new tubes installed. I used the original T piece, although a new one came with the tubing. The original was a firmer fit.
The next image is a better view of the leather apron which covers the back of the instruments and circuit board.
Finally I tested the system and there were no leaks.