A few months ago, I noticed a leak from the washer tank when I filled it nearly to the top of the filler tube. At the time, I let it go. Then about two months ago, the pump seized on me, and I replaced it with an OE pump last month. Since the replacement, the leak seemed to have gotten worse, and I generally had to top up every week or so. I finally removed the tank and figured out what was going on. First, the worse leak was happening because the rubber sleeve and gasket that seal the pump intake opening had managed to fall into the tank while I was monkeying around with the pump. Without that seal in place, you lose about 80% of the tank's capacity (the sleeve not only seals, but also extends the pump's intake so that it reaches the bottom of the tank). I had to use a pair of chopsticks to fish it out. The initial, smaller leak, is just happening because there isn't an effective seal between the filler tube and the tank.

You can check for the rubber sleeve by pulling out the pump from inside the engine compartment. The sleeve should come out with the pump. If it doesn't, just take a look at the opening where the pump was.

Does anybody know if there's supposed to be an o-ring or something around the filler tube connection?