Does anyone know how these original AC vacuum canisters hold up over time? I'm always finding ways to save money where I can. I believe the newer tanks I can buy are plastics and this one seems pretty solid.

 

Pull a vacuum on it and see if it leaks. Some small leakage is tolerable. The main purpose of the vacuum canister is to provide a vacuum reservoir during low-vacuum engine operation (ie, wide open throttle), typically going up a hill.