I'm not sure what it is but based on it's position and what it's hooked to, I"m guess a vacuum canister. Your power brake system relies on engine vacuum to assist with reducing how hard you have to push on your brakes. Try pushing the pedal with the engine off and you'll see the difference. I've seen V8 versions for guys who run high lift/long duration cams (the old muscles cars that barely seem to be able to idle) and therefore don't generate much engine vacuum. They will use a vacuum canister (might not be the right word, neve rmuch of a muscle car guy mysefl) to basically buffer and store the vacuum pressure and help the power brake system work properly even when the engine wasn't helping.

As for what it's doing on your car, my guess is the previous owner was having a brake issue and someone told him to install one to help out. On the factory honda line there is a white check valve that only allows air to flow one way. If the guy removed the facotry line and replaced it with one that did not have a check valve it may have caused the system to work improperly and so someone told him he needed a vacuum canister, which most likely would have a check valve built in. It was overkill but worked so it was left on. That or the guy had an extra cherry bomb glass pack and said "I'll bet this would look bitching under the hood, it's even red!". It's definately not an OEM piece I've ever seen.

Does the car have upgraded brakes. My old CX with it's 13/16 MC couldn't push enough fluid to properly operate a set of GS-R front calipers. The pedal would go to the floor like there was air in the system but it was bleed (that MC also had 150k miles on it). I replaced the MC with a 1" ABS GS-R unit and it worked fine. Maybe he tried the vacuum canister rather than upgrading the MC?