[Sparkee]

In my case, the tank would slowly leak down to empty. So, expansion of the fluid clearly wasn't the issue. After pulling the tank, I hung the pump between two elevated hoses holding fluid/water, one still attached on the washer jet side the other a hose I had laying around. I could see the water leaking out of the side of the pump at a rate of about a drip every 5 seconds with this arrangement. It turned out to be a good way to prove it was the pump.

I am glad Danny's fix worked too...