power brake boosters

[Tom Jennings <tomj@xxxxxxx>]

So what's the deal with power brake boosters? I have very little  
experience with them (few of my cars have had power brakes!) and I've  
got a bad one; it leaks down less than 20 seconds after the engine's  
off, and it has a "stiff spot" (low assist) on the first bit of  
travel (feels like "stiction" in the pedal or linkage, but I checked  
very carefully).

The only replacement I can find is $110 from NAPA, and comes with a  
master cylinder. I don't need one of those, I ahve a new one installed.

Are boosters generally interchangable? Are they rebuildable at home?  
Kits?

I suspect there's only two or three physical types (in AMC land) but  
I have no way to identify them.