550 brake locked at the engine start

I had experienced the brake locked three times during the last 12 months at the engine start in the morning. Since I don't drive my 2002 550 much, I usually turn off the battery connection. About 12 months ago, I turned on the battery connection and start the engine. The brake was locked (completely forzen) and I could not step on it. After turning off the engine and battery connection and waiting 10 min, I re-stared the engine and, this time, the brake worked well. The same happened a couple of times after that incient. I am not sure what has caused it. I may not have followed the exact engine start procedure as I may not have waited for "Checked OK" signal. I am not sure if it has caused the issue. Or should I just replace the brake system? This seems a lot more series issue.