$700 later, and the leak in the sprinkler feed line is fixed, which has resulted in discovering the other leak in the sprinkler feed line that is the one actually causing the problem.
We turned the sprinklers on a few weeks ago, and our basement promptly flooded. Turned the sprinklers off.
Water gets to our sprinkler system by a pipe coming out of the house. The pipe comes out of the foundation, through a small flower bed, and then through a low concrete wall surrounding the flower bed, and finally into the sprinkler box with the manifold. We dug out the flower bed to reveal the pipe, pressurized the sprinklers, and found a leak where the pipe went into the concrete wall.
Got that leak repaired today, went into the basement and saw water starting to come out of the floor. Didn’t seem like that much leaked during testing… Went out and dug out the other end of the pipe, near the foundation, turned the sprinklers on, and sure enough, water is coming out of the foundation around the pipe.
Next step is to find where the pipe enters the foundation in the basement ceiling. It is a finished basement, so this will involve measuring, guessing, and carefully removing 1970s fake wood paneling.
Also, when I pressurized the sprinklers I forgot to close the drain valve, so the floor in the furnace room flooded. Sucked up 6 gallons of water with the shop vac, which was enough for me to clean the spiders off the floor drain so the rest of the water could flow out.
Oh yeah, and the basement is where my wife keeps most of her horde, so while this is all going on I’m having to climb around plastic tubs filled with yarn, fabric, and for some reason IBM electric typewrites.