power brake booster pump

first of all, thanks in advance for any help and thanks just for being here, i appreciate all the input i have recieved in the past. replacing my entire a/c
was a chore but i got it done, but somehow broke something (a vent door for the floor vents or something) so every time i start or stop the engine or push on or off the recirculate button i get about 12 seconds of clicking noise from the vent, but i can live with that considering its a 10 yr old with 220K.
now to my sons truck - '96 350 Z-71
brake pedal is very mushy and has a "air suction" sound. holding a baggie around the brake rod grommet at the firewall shows that air is passing from the cab into the firewall, assumedly into the booster pump. so - assumedly this needs replacing.
having never messed with these before - any suggestions? questions:
are there any tricks to removal and replacement, or is it just straightforward? is a brake bleeding necessary afterwards? is a used part worht the savings? any way to tell if i should replace the master cylinder also, or is it recommended to go ahead and do it anyway?
also, i glanced through All Data DIY and didnt see much in there about the booster pump but i really didnt have time to look too hard last night, and chiltons has nothing. does this mean it is too much for a DIYer?
david
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ok, after doing some reading i realize that a bad booster would result in a harder brake pedal, not mushier. it must be normal for there to pe air passing from the cab through the firewall - this is the air that powers the booster (obviously i knew zilch about this until today)
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The VACUUM that powers the booster comes from a big old hose from the intake manifold over to the booster.
Mushy pedals: Worn brake linings, expanding flex lines, low fluid, dirty fluid.....
GMC Gremlin

a
passing
booster
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accidentally sent the previous post before finishing -
so now i guess i should replace the master cylinder since there are no apparent leaks elsewhere in the system. i just always thought that master cylinder failure was evident by leackage arounf the cylinder.
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On 12 Oct 2004 17:15:56 GMT, snipped-for-privacy@aol.com (Surfgeotwo) wrote:

That's not the case. MC in my old truck was allowing fluid to pass by the piston and bypass (only partially though) the rear brake ports. No leakage.
The air passing through the firewall is actually filling the backside of the booster I guess. The actual brake boost is the vacuum line hooked to the front of the booster...
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