I followed the following. I recently replaced my cap and when I opened it there was a hiss of release of pressure.
Probable solution:
windshield washer nozzles leak - Bimmerfest - BMW Forums
Ok Folks, this problem is SOLVED!! I was sitting at the indie shop getting ac charged, reading about Dane's son's continuing saga......so i saw the shop owner and I said hey, Barry, got a second for a brian teaser? I desribed my problem to him, and how much time ive spent on it. He started smiling before I could finish the story. Immediately he says-------"You've got a clogged vent on the washer fluid tank somewhere. The tank has to vent because it heats up, and the air inside expands. If it doesnt vent, it forces the fluid through the tubes to the nozzles."
Ha! makes perfect sense. I had replaced the fluid cap (with on from BMW) but the old one must have had a venting mechansm or something on it. Solution- drill a 1/16 hole in the center of the washer fluid cap. I wasted 2 cases of beer and countless hours on this, and the solution was right there......Should have drank less and pondered more! lol.