456M brake booster hissing noise

My 456M brake power is subpar and response is not good. After completely flushed 4 calipers and master cylinder, it has no improvement. I hear hissing noise every time I pump the brake. (seems like it is a common problem for european cars according to many google search) I found there is NO seal nor dust cover around the plunger, quite sure the air is leaking from the plunger. See pics. (I also attached a pic found on internet has some seal left around the plunger)

Thinking about rebuilding the booster and master cyl (Part # 177034) Anyone has rebuilt the brake boost/master cyl. before?

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