Check Brake System Warning

Hey all, Happy Easter.

Just had a random question for you guys, just wanted to see whether anyone else has had this happen to them. So when at idle, after I had the drums off and was painting the drums, I messed with the shoes a little bit while also discovering I had a torn wheel cylinder boot. Anyway, ordered replacement advics units to replace them. They aren’t leaking now but want to do it and get it out of the way, so I don’t have to worry about it in the future. Anyway, I press the brake pedal several times and rapidly, a good bit into its travel, and then I pushed it down far enough, but it didn’t feel far, and then the check brake system light came on. Mind you, this is all at idle. Then I rev it up and start driving and it’s all back to normal. I was thinking this was because the engine can’t produce enough vacuum for the booster at idle, which would explain why it changes when the engine revs. Any thoughts? Thanks.