Power brake boster?
 

My neighbor gave me a power brake booster off a 69 3/4 ton 4-wheel drive the master cylinder is rusted bad left out in weather with no cap. My question is can I use the master cylinder I have now and bolt in on the booster and also how do I check to see if the booster is good?

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67 GMC Stepside(UNDER CONSTRUCTION)
67 GMC Custom
67 Chevy C-30

booster is good if it holds vacuum in it enough to push pedal 2 or 3 times before using up the reserve and making the pedal hard again, this is when engien is off after just being run and shut off

you should be able to use any power master cylinder on any booster, *should*
compare the mounting area of the two cylinders to make sure the cylinder protrudes/is sunk into the casting body of the cylinder the same as the other, manual brake cylinder wont work on a booster