I tend to disagree with humy here
I have seen a few engines where vaccum is used to speed up the engine (nissan) when ac operates
even to close and open inside ducts (daewoo)
so if vaccum pipes are messed up brake performance will be effected as the same vaccum is used for the power brakes(if somebody doesnt know)
checking all vaccum pipe would be a good idea
genrally when engine races up brake performance is improved as more vaccum is produced
i dont think crown engine would race up very much on ac operation
depends on engine size / type