allon wrote:If it is boiler water, then you've got bigger problems.
I'm at a loss. Perhaps you don't understand the function of the vacuum breaker....
This valve is mounted to the boiler, and communicates a pathway from inside to outside the boiler. When the valve is OPEN, then there is no steam because the boiler is cold, or just starting to heat up. The valve is normally open, with a small piston held open by a spring.
As the boiler heats, the valve allows air+water to escape until the boiler gets hot enough to pressure the valve and seal it shut by forcing the piston against the seal. At this point, there is some water (droplets) trapped above the seal, and so as the boiler heats further then that water is evaporated.
This cycle is repeated every time the machine is started up, so what happens is small amounts of water get trapped and evaporated repeatedly, leaving behind deposits. Even very clean water has some disolved solids, so unless one fills the steam boiler with distilled water then the breaker will eventually need cleaning.