Classic/Antique Car Repair: Power Brake Booster, power brake booster, brake job

Question
I took apart a power booster on a custom royal dodge 1956.  It seems ok but i can't really tell why it's not working.  I spent $700 on a brake job and the mechanic suggests i need it to be rebuilt.  I am confused since when i took this off, it's not rusted, bent, broken or in bad shape at all.  It looks like it was replace about 3-7 years ago from the looks of it.  Not sure how to test it without sending it in and having someone tell me it needs a total over haul for $350 when it doesn't even look worn?  Air goes through hoses and pistons seem to be straight and sturdy.  HELP?  Eli

Answer
The test that we have used for years when checking the operation of a vacuum operated power brake system is as follows:
1. With the engine off pump the brake pedal a half a dozen times to remove all vacuum from the booster.
2. Apply light but firm pressure to the brake pedal. While holding the pedal down start the engine. If the booster is working the pedal will move down under your foot. If it does it is working and if it does not it is not working.
The thing that goes in these systems after 50 years is the valve that controls the flow of vacuum into and out of the cylinder. Sounds like you might put the unit back together and test it to see if it works. Why did the mechanic suspect the vacuum brake booster?
Brad