I couldn't say that it was exactly 20 minutes but it was an interminable inferno of screeching, dash lights, eventual alarm, and finally failure of power braking. I was still able to stop but there wasn't power boost and they were greatly reduced. All this would pass after an amount of time...
My 2000 LC started making the dying seagull screech after rain. Also on mornings with a lot of dew. Started out brief and infrequent and increased over a year or two to 20 minutes non-stop and eventually complete power brake failure a few times.
It seemed to me that the screeching stopped after...