| The device that holds the pressure is called an overflow valve. Typically this valve only passes pressure beyond it after brake (and other critical) system pressure is up to 5 bar or so. 4 bar sounds reasonable, too, because it's an adjustable device. If there's a pressure leak 'beyond' the overflow valve then pressure will leak down to the 5 or 4 bar and hold. BTW, I have three 406's and a 1300 that do this. I have considered this normal behavior and don't worry about it.
Bob |