Also low fluid won't cause spongy feeling. I once got a low fluid light from parking nose down but wasn't low enough to get air into the system. If you can push the pedal lower once it firms up either there is air or something rubber is expanding such as the brake hose. If somehow managed to fail in a way it inflated like a balloon but didn't break.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)