It is simple logic, if you have a valve operating a device with vacuum then when the vacuum is no longer required air is let in to the device. This air has to be clean or the device will fill with dirt and stop working. So you must have a filter on all valves, The easiest one to see is on the brake booster. Eric.
On my 2003 the vacuum vent filter is behind the heater control valve. A long hose connects it to the vacuum transducer (boost actuator) under the airbox. It's mounted vertically with the hose on the top. You can track it down by getting under the hood immediately after turning the engine off and listening for a quiet sucking sound. Someone on this forum suggested that any small engine fuel filter would work as a replacement, since only a small amount of air flows through it. I imagine that it is a maintenance item, like any filter.
No, that is the normal pedal feel. Madness, but normal.
You can reassure yourself the system is free of air by stomping on the brakes a few times before starting the engine: you’ll quickly get a firm pedal with no sponginess (unless your brakes need bleeding). Then hold the pedal and start the engine, and feel the pedal turn to mush as it drops another inch or two under boost assist.
Thank you, Mother Benz.
(maybe it was to make drivers used to air brakes feel more at home?)
Added: if your brakes are FADING as you drive, your pads may be thin or the fluid may be old and wet, boiling in the calliper during continuous braking?