Question
Can you get a hard stable pedal with the engine off?
If yes and the pedal drop only happens with the engine running, then look up stream from the master and booster, vac line, etc
I can not tell what your year, make, model is as there is nothing in your signature line
On the 4.8is application there is a right side cam driven vacuume pump, when it is dry for an extended period of time it fails
See realoem for exploded visuals
On theses cars, a bad booster, vac leaks will give you a soft pedal engine on!

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2005 X5 4.8IS
The Blue ones are always FASTER....

Current Garage:
2005 X5 4.8is
2002 M5 TiSilver
2003 525iT
1998 528i
Former Garage Stable Highlights
2004 325XiT Sport
1973 De Tomaso Pantera, L Model
1970 Dodge Challenger T/A 4 sp Alpine White
1970 Dodge Challenger T/A 4 sp GoManGo Green
1971 Dart Sport, “Dart Light” package
1969 Road Runner 383
1968 Ply Barracuda 340S FB Sea-foam Green